Readme: document using vim/ as an rtp

Some plugins have repos that host multiple versions of the plugin, with the vim version living in `/vim` or similar. Some examples:

* http://github.com/chriskempson/tomorrow-theme/tree/master/vim
* http://github.com/rstacruz/sparkup/tree/master/vim
* https://github.com/rust-lang/rust/tree/master/src/etc/vim (gasp)

pathogen actually can support these with a bit of a hack. This documents that hack.