Document how to get diff stats in statusline

Closes #674.

README.mkd

@@ -234,6 +234,20 @@ gitgutter#fold#foldtext():  +-- 45 lines (*): abcdef
 You can use `gitgutter#fold#is_changed()` in your own `foldtext` expression to find out whether the folded lines have been changed.
 
 
+### Status line
+
+Call the `GitGutterGetHunkSummary()` function from your status line to get a list of counts of added, modified, and removed lines in the current buffer.  For example:
+
+```viml
+" Your vimrc
+function! GitStatus()
+  let [a,m,r] = GitGutterGetHunkSummary()
+  return printf('+%d ~%d -%d', a, m, r)
+endfunction
+set statusline+=%{GitStatus()}
+```
+
+
 ### Customisation
 
 You can customise:

doc/gitgutter.txt

@@ -17,6 +17,7 @@ CONTENTS                                                            *gitgutter*
   Commands ..................... |gitgutter-commands|
   Mappings ..................... |gitgutter-mappings|
   Autocommand .................. |gitgutter-autocommand|
+  Status line .................. |gitgutter-statusline|
   Options ...................... |gitgutter-options|
   Highlights ................... |gitgutter-highlights|
   FAQ .......................... |gitgutter-faq|
@@ -244,6 +245,23 @@ trailing empty lines.
 <
 
 
+===============================================================================
+STATUS LINE                                              *gitgutter-statusline*
+
+
+Call the `GitGutterGetHunkSummary()` function from your status line to get a
+list of counts of added, modified, and removed lines in the current buffer.
+For example:
+>
+    " Your vimrc
+    function! GitStatus()
+      let [a,m,r] = GitGutterGetHunkSummary()
+      return printf('+%d ~%d -%d', a, m, r)
+    endfunction
+    set statusline+=%{GitStatus()}
+<
+
+
 ===============================================================================
 OPTIONS                                                     *gitgutter-options*