Instructions for editing the spec

With a massive amount of help from David C and Deyan, there is now a wiki
page <https://github.com/w3c/mathml/wiki/Workflow-for-working-on-the-spec>
on the MathML github page with instructions on how to use git to work with
the spec. There are also instructions for how to view your changes locally
before you push them back and do a "Pull Request".

We hope this makes it easier for everyone to contribute and also to avoid
mistakes (like I made this week).