Reason for Change
=================
* As of [this commit], themes are now in a separate repository.
* The `README.md` file does not reflect this change as of yet.
[this commit]: https://github.com/vim-airline/vim-airline/commit/d7fd8ca649e441b3865551a325b10504cdf0711b.
Changes
=======
* Mention the external themes when discussing them.
* Create a new "Themes" sub-section and add information on how to add them.
Note: this addresses Issue #981.
