Update readme
Update readme

The vim plugin manager wars are too intense for keeping dedicated
snippets in each plugin's readme. User's should know how to install
plugins with their manager of choice. If not, provide an example. I
would prefer to send people to an up to date page about this.
Unfortunately I don't think this currently exists.

aclissold/swift.vim,ajh17/swift.vim,keith/swift.vim,michaelnisi/swift.vim,mokagio/swift.vim,gfontenot/swift.vim,panghea/swift.vim