Cleaned up contributing guidelines

[Andy Parker]

In an effort to streamline and consolidate how code gets submitted to
puppet, we've updated the contributing guidelines. The changes were
along three fronts:

1. Clarify what to target when submitting patches.
2. Reflect the reality that we take code submissions as github pull requests.
3. Simplify the explanation for how to get up and running as a contributor.

We are also in discussions with our lawyers about whether the CLA is
still needed, but there has been no decision about that yet.

The CONTRIBUTING.md document is now a lot shorter, and the longer form
has been moved off to the wiki. You can read it at
https://github.com/puppetlabs/puppet/blob/master/CONTRIBUTING.md

If you have any questions, feel free to ask.

Thanks,
Andrew Parker
Puppet/Facter Team Lead

[Jeff McCune]

And if you want to get a little meta, an example pull request against the contributing document itself is at:

https://github.com/puppetlabs/puppet/pull/1001

This pull request gives an example of the topic branch name, separating commits out into logical commits, example commit messages, cleaning up whitespace with `git diff --check`, etc...  These are all of the housekeeping things that can be a pain when first getting started with Git, but become much easier and almost thoughtless with a bit of practice.  This housekeeping stuff is important because it makes it easier and faster to read and work with changes.

-Jeff