Update Vim usage instructiosn.
Update Vim usage instructiosn.

- Link to pick.vim instead of supplying a `PickCommand()` function.
- Add a note about GUI incompatibility.

prahlad37/pick,thoughtbot/pick,temochka/pick,GBuella/pick,DBOTW/pick,calleerlandsson/pick,calleerlandsson/pick,GBuella/pick,thoughtbot/pick,prahlad37/pick,thoughtbot/pick,DBOTW/pick,DBOTW/pick,calleerlandsson/pick,temochka/pick,prahlad37/pick,thoughtbot/pick,prahlad37/pick