Add instructions on how to customize colours in preview window

README.mkd

@@ -283,6 +283,9 @@ You can customise:
 * How to handle non-gitgutter signs
 * The signs' colours and symbols
 * Line highlights
+* Line number highlights (only in Neovim 0.3.2 or higher)
+* The diff syntax colours used in the preview window
+* The intra-line diff highlights used in the preview window
 * Whether the diff is relative to the index (default) or working tree.
 * The base of the diff
 * Extra arguments for `git` when running `git diff`
@@ -395,6 +398,35 @@ highlight link GitGutterChangeLineNr Underlined
 ```
 
 
+#### The diff syntax colours used in the preview window
+
+To change the diff syntax colours used in the preview window, set up the `diff*` highlight groups in your colorscheme or `~/.vimrc`:
+
+```viml
+diffAdded   " if not set: use GitGutterAdd's foreground colour
+diffChanged " if not set: use GitGutterChange's foreground colour
+diffRemoved " if not set: use GitGutterDelete's foreground colour
+```
+
+Note the `diff*` highlight groups are used in any buffer whose `'syntax'` is `diff`.
+
+
+#### The intra-line diff highlights used in the preview window
+
+To change the intra-line diff highlights used in the preview window, set up the following highlight groups in your colorscheme or `~/.vimrc`:
+
+```viml
+GitGutterAddIntraLine    " default: gui=reverse cterm=reverse
+GitGutterDeleteIntraLine " default: gui=reverse cterm=reverse
+```
+
+For example, to use `DiffAdd` for intra-line added regions:
+
+```viml
+highlight link GitGutterAddIntraLine DiffAdd
+```
+
+
 #### Whether the diff is relative to the index or working tree
 
 By default diffs are relative to the index.  How you can make them relative to the working tree: