In README, improve summary and instructions

- add “Vim” so people know that this is only for Vim, if they got here by search
- the extra hyphens make parsing the sentence a bit easier
- Add period to make the sentence grammatical. I don’t think anyone will accidentally include the period in the command they type, since the command is formatted differently.
- Warn about [issue #1](https://github.com/dahu/bisectly/issues/1), that Bisectly won’t work on gVim, so people don’t waste their time trying it in gVim.