Separation of CLA and CONTRIBUTING

[jmakeig]

The CLA provides the one-time legal form to allow someone to contribute. CONTRIBUTING is a document that describes the process/best practice for an external contributor to submit a pull request. (It's actually linked to in the PR UI.) This task is to separate the current implementation as it is today.

[sammefford]

Justin, can you confirm the changes to Contributing.md I merged from David Cassel's pull request fulfill this task?