document git commit error

Addresses #23 however a strictly Vim solution would be preferable ultimately.

Add documentation on how to correct the error
```
error: There was a problem with the editor 'vi'.
Please supply the message using either -m or -F option.
```
Solution comes from
http://tooky.co.uk/there-was-a-problem-with-the-editor-vi-git-on-mac-os-x/