Add option to disable asynchronous diffs.

Closes #318.

doc/gitgutter.txt

@@ -126,6 +126,7 @@ You can customise:
 - Whether or not line highlighting is on initially (defaults to off)
 - Whether or not vim-gitgutter runs in realtime (defaults to yes)
 - Whether or not vim-gitgutter runs eagerly (defaults to yes)
+- Whether or not vim-gitgutter runs asynchronously in NeoVim (defaults to yes)
 
 Please note that vim-gitgutter won't override any colours or highlights you've
 set in your colorscheme.
@@ -266,6 +267,15 @@ Add to your |vimrc|
   let g:gitgutter_eager = 0
 <
 
+TO TURN OFF ASYNCHRONOUS UPDATES
+
+By default diffs are run asynchronously in NeoVim.  To run diffs synchronously
+instead:
+
+Add to your |vimrc|
+>
+let g:gitgutter_async = 0
+<
 
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 6. FAQ                                                           *GitGutterFAQ*