add git commands for submitting pull request

I am not sure how much detail is appropriate here (or if there should be a FAQ as in the bc/README.md), but these are some notes I took when helping a new git user submit a pull request.

I also removed redundant text about "if it is easier ... please mail us"
Use

    ~~~
    $ git clone git@github.com:<your-username>/bc.git
    ~~~

instead.