Document cmd.exe vulnerability where it can execute the wrong git.

See #508, #538.

README.mkd

@@ -104,6 +104,17 @@ cp -r vim-gitgutter/* ~/.vim/
 See `:help add-global-plugin`.
 
 
+### Windows
+
+I recommend configuring vim-gitgutter with the full path to your git executable.  For example:
+
+```viml
+let g:gitgutter_git_executable = 'C:\Program Files\Git\bin\git.exe'
+```
+
+This is to avoid a problem which occurs if you have file named `git.*` (i.e. with any extension in `PATHEXT`) in your current folder.  `cmd.exe` prioritises the current folder over folders in `PATH` and will try to execute your file instead of the `git` binary.
+
+
 ### Getting started
 
 When you make a change to a file tracked by git, the diff markers should appear automatically.  The delay is governed by vim's `updatetime` option; the default value is `4000`, i.e. 4 seconds, but I suggest reducing it to around 100ms (add `set updatetime=100` to your vimrc).

doc/gitgutter.txt

@@ -79,6 +79,19 @@ Copy vim-gitgutter's subdirectories into your vim configuration directory:
 See |add-global-plugin|.
 
 
+WINDOWS                                                     *gitgutter-windows*
+
+I recommend configuring vim-gitgutter with the full path to your git executable.
+For example:
+>
+    let g:gitgutter_git_executable = 'C:\Program Files\Git\bin\git.exe'
+<
+This is to avoid a problem which occurs if you have file named "git.*" (i.e.
+with any extension in "PATHEXT") in your current folder.  "cmd.exe" prioritises
+the current folder over folders in 'PATH' and will try to execute your file
+instead of the "git" binary.
+
+
 ===============================================================================
 COMMANDS                                                   *gitgutter-commands*