Closes #1050. Update README; don't show NERDTree if opening a session.

README.markdown

@@ -17,34 +17,62 @@ included documentation.
 Installation
 ------------
 
+Below are just some of the methods for installing NERDTree. Do not follow all of these instructions; just pick your favorite one. Other plugin managers exist, and NERDTree should install just fine with any of them.
+
+#### Vim 8+ packages
+
 If you are using VIM version 8 or higher you can use its built-in package management; see `:help packages` for more information. Just run these commands in your terminal:
 
-    git clone https://github.com/scrooloose/nerdtree.git ~/.vim/pack/vendor/start/nerdtree
+```bash
-    vim -u NONE -c "helptags ~/.vim/pack/vendor/start/nerdtree/doc" -c q
+git clone https://github.com/scrooloose/nerdtree.git ~/.vim/pack/vendor/start/nerdtree
+vim -u NONE -c "helptags ~/.vim/pack/vendor/start/nerdtree/doc" -c q
+```
 
-Otherwise, these are some of the other options:
+Otherwise, these are some of the several 3rd-party plugin managers you can choose from. Be sure you read the instructions for your chosen plugin, as there typically are additional steps you nee d to take.
 
 #### [pathogen.vim](https://github.com/tpope/vim-pathogen)
 
-    git clone https://github.com/scrooloose/nerdtree.git ~/.vim/bundle/nerdtree
+In the terminal,
+```bash
+git clone https://github.com/scrooloose/nerdtree.git ~/.vim/bundle/nerdtree
+```
+In your vimrc,
+```vim
+call pathogen#infect()
+syntax on
+filetype plugin indent on
+```
 
-Then reload Vim, run `:helptags ~/.vim/bundle/nerdtree/doc/` or `:Helptags`, and check out `:help NERDTree.txt`.
+Then reload vim, run `:helptags ~/.vim/bundle/nerdtree/doc/` or `:Helptags`.
 
+#### [Vundle.vim](https://github.com/VundleVim/Vundle.vim)
+```vim
+call vundle#begin()
+Plugin 'scrooloose/nerdtree'
+call vundle#end()
+```
+
+#### [vim-plug](https://github.com/junegunn/vim-plug)
+```vim
+call plug#begin()
+Plug 'scrooloose/nerdtree'
+call plug#end()
+```
 
 #### [apt-vim](https://github.com/egalpin/apt-vim)
+```bash
+apt-vim install -y https://github.com/scrooloose/nerdtree.git
+```
 
-    apt-vim install -y https://github.com/scrooloose/nerdtree.git
+F.A.Q. (here, and in the [Wiki](https://github.com/scrooloose/nerdtree/wiki))
-
-F.A.Q.
 ------
 
-> Is there any support for `git` flags?
+#### Is there any support for `git` flags?
 
 Yes, install [nerdtree-git-plugin](https://github.com/Xuyuanp/nerdtree-git-plugin).
 
 ---
-
+#### Can I have the nerdtree on every tab automatically?
-> Can I have the nerdtree on every tab automatically?
 
 Nope. If this is something you want then chances are you aren't using tabs and
 buffers as they were intended to be used. Read this
@@ -53,53 +81,65 @@ http://stackoverflow.com/questions/102384/using-vims-tabs-like-buffers
 If you are interested in this behaviour then consider [vim-nerdtree-tabs](https://github.com/jistr/vim-nerdtree-tabs)
 
 ---
-> How can I open a NERDTree automatically when vim starts up?
+#### How can I open a NERDTree automatically when vim starts up?
 
 Stick this in your vimrc: `autocmd vimenter * NERDTree`
 
 ---
-> How can I open a NERDTree automatically when vim starts up if no files were specified?
+#### How can I open a NERDTree automatically when vim starts up if no files were specified?
 
 Stick this in your vimrc:
-
+```vim
-    autocmd StdinReadPre * let s:std_in=1
+autocmd StdinReadPre * let s:std_in=1
-    autocmd VimEnter * if argc() == 0 && !exists("s:std_in") | NERDTree | endif
+autocmd VimEnter * if argc() == 0 && !exists("s:std_in") | NERDTree | endif
+```
 
 Note: Now start vim with plain `vim`, not `vim .`
 
 ---
-> How can I open NERDTree automatically when vim starts up on opening a directory?
+#### What if I'm also opening a saved session, for example `vim -S session_file.vim`? I don't want NERDTree to open in that scenario.
+```vim
+autocmd StdinReadPre * let s:std_in=1
+autocmd VimEnter * if argc() == 0 && !exists("s:std_in") && v:this_session == "" | NERDTree | endif
+```
 
-    autocmd StdinReadPre * let s:std_in=1
+---
-    autocmd VimEnter * if argc() == 1 && isdirectory(argv()[0]) && !exists("s:std_in") | exe 'NERDTree' argv()[0] | wincmd p | ene | exe 'cd '.argv()[0] | endif
+#### How can I open NERDTree automatically when vim starts up on opening a directory?
+```vim
+autocmd StdinReadPre * let s:std_in=1
+autocmd VimEnter * if argc() == 1 && isdirectory(argv()[0]) && !exists("s:std_in") | exe 'NERDTree' argv()[0] | wincmd p | ene | exe 'cd '.argv()[0] | endif
+```
 
 This window is tab-specific, meaning it's used by all windows in the tab. This trick also prevents NERDTree from hiding when first selecting a file.
 
 Note: Executing `vim ~/some-directory` will open NERDTree and a new edit window. `exe 'cd '.argv()[0]` sets the `pwd` of the new edit window to `~/some-directory`
 
 ---
-> How can I map a specific key or shortcut to open NERDTree?
+#### How can I map a specific key or shortcut to open NERDTree?
 
 Stick this in your vimrc to open NERDTree with `Ctrl+n` (you can set whatever key you want):
-
+```vim
-    map <C-n> :NERDTreeToggle<CR>
+map <C-n> :NERDTreeToggle<CR>
+```
 
 ---
-> How can I close vim if the only window left open is a NERDTree?
+#### How can I close vim if the only window left open is a NERDTree?
 
 Stick this in your vimrc:
-
+```vim
-    autocmd bufenter * if (winnr("$") == 1 && exists("b:NERDTree") && b:NERDTree.isTabTree()) | q | endif
+autocmd bufenter * if (winnr("$") == 1 && exists("b:NERDTree") && b:NERDTree.isTabTree()) | q | endif
+```
 
 ---
-> Can I have different highlighting for different file extensions?
+#### Can I have different highlighting for different file extensions?
 
 See here: https://github.com/scrooloose/nerdtree/issues/433#issuecomment-92590696
 
 ---
-> How can I change default arrows?
+#### How can I change default arrows?
 
 Use these variables in your vimrc. Note that below are default arrow symbols
-
+```vim
-    let g:NERDTreeDirArrowExpandable = '▸'
+let g:NERDTreeDirArrowExpandable = '▸'
-    let g:NERDTreeDirArrowCollapsible = '▾'
+let g:NERDTreeDirArrowCollapsible = '▾'
+```