Add a note about unstaging staged changes.

README.mkd

@@ -110,6 +110,8 @@ You can stage or revert an individual hunk when your cursor is in it:
 * stage the hunk with `<Leader>hs` or
 * revert it with `<Leader>hr`.
 
+See the FAQ if you want to unstage staged changes.
+
 The `.` command will work with both these if you install [repeat.vim](https://github.com/tpope/vim-repeat).
 
 To set your own mappings for these, for example if you prefer the mnemonics hunk-add and hunk-undo:
@@ -281,6 +283,24 @@ Add `let g:gitgutter_highlight_lines = 1` to your `~/.vimrc`.
 
 ### FAQ
 
+> Why can't I unstage staged changes?
+
+Unstaging staged hunks is feasible but not quite as easy as it sounds.  There are three relevant versions of a file at any one time:
+
+1. The version at HEAD in the repo.
+2. The version staged in the index.
+3. The version in the working tree, in your vim buffer.
+
+`git-diff` without arguments shows you how 3 and 2 differ; this is what vim-gitgutter shows too.
+
+`git-diff --staged` shows you how 2 and 1 differ.
+
+Let's say your are looking at a file in vim which has some unstaged changes.  Now you stage a hunk, either via vim-gitgutter or another means.  The hunk is no longer marked in vim-gitgutter because it is the same in 3 and 2.
+
+Now you want to unstage that hunk.  To see it, you need the difference between 2 and 1.  For vim-gitgutter to shows those differences, it would need to show you 2 instead of 3 in your vim buffer.  But 2 is virtual so vim-gitgutter would need to handle it without touching 3.
+
+I intend to implement this but I can't commit to any deadline.
+
 > Why are the colours in the sign column weird?
 
 Your colorscheme is configuring the `SignColumn` highlight group weirdly.  Please see the section above on customising the sign column.