document git commit error

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/
Merge pull request #32 from salcode/document-vim-git-commit-fix

document git commit error