Add explanation and workaround of current release (0.9) being unable to show current git branch with latest release of vim-airline (2.4.)

FAQ.md

@@ -81,7 +81,11 @@ Certain themes are derived from the active colorscheme by extracting colors from
 
 #### You do not see the current Git branch
 
-You need [fugitive](https://github.com/tpope/vim-fugitive) to see the current git branch. If you have it and still don't see it then you are probably using version 1.2, which is very old.  vim-airline relies on a `fugitive#head()` function.  Download v2 from the [project page][d].
+You need [fugitive](https://github.com/tpope/vim-fugitive) to see the current git branch. If you have it and still don't see it then one of two things is happening:
+
+1. You have too old of a version of vim-fugitive.  vim-airline relies on a `fugitive#head()` function, so you should download a new version from [fugitive's project page][d].  If you are running release 0.9.0 of vim-airline, rather than a recent git commit, you shouldn't upgrade vim-fugitive past v2.3, or you'll run into the next potential problem.
+
+2. You have too new of a version of vim-fugitive, and are running the latest release of vim-airline rather than a git version.  [tpope/vim-fugitive@5d11ff7](https://github.com/tpope/vim-fugitive/commit/5d11ff7) made `fugitive#head()` autoloadable, which broke the latest 0.9.0 release of vim-airline.  The next release of vim-airline is expected in December (see [#1820](https://github.com/vim-airline/vim-airline/issues/1820)), and the fix will not be backported (see [#1815](https://github.com/vim-airline/vim-airline/issues/1815)).  Until then, you either need to downgrade vim-fugitive to v2.3, or run a recent git commit version of vim-airline.
 
 #### vim-bufferline is printing to the statusline as well as the command bar