Update README.markdown

Added information from this forum topic on how to add a bundle to the list: https://groups.google.com/forum/#!searchin/spf13-vim-discuss/install/spf13-vim-discuss/s_PMWxVVogM/otCEQs0ZlsYJ
Merge pull request #675 from jaguarondi/patch-1

Update README.markdown
Merge remote-tracking branch 'upstream/3.0' into 3.0