Plowshare: Add caveat about Bash 4+ requirement

Plowshare 4 requires Bash 4+ to run. OS X ships with an old Bash version 3, so the user must install a brewed version of Bash before they are able to run Plowshare 4.

As installing a new shell is something that we shouldn't do on our own, I propose the addition of a small caveat stating that Bash 4 is required, and how it can be install from Homebrew.

Plowshare 3, which is still supported and maintained (and it runs with default OS X Bash) is available at my own tap:

```sh
brew tap danielmartin/homebrew-plowshare
brew install plowshare3
```

But it could also be added to homebrew-versions, just for people who don't want to install a new shell.

What do you think?