New preferences for MIT krb5 code contributions

[ghudson@MIT.EDU]

We've written up new guidelines for code contributions at:

    http://k5wiki.kerberos.org/wiki/Contributing_code

The short summary is:

* We now prefer (but don't require) receiving contributions as github
  pull requests.

* We require one logical change per commit on the pull request branch
  (or per patch if not using git).

* For larger contributions, we require a project proposal (which acts
  as a design document), automated tests, and documentation.  In some
  cases we can do the work of meeting those requirements, but it will
  usually delay accepting the code contribution.