Continuous integration (rebased onto develop)



This is the same as gh-196 but rebased onto develop.

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This PR converts the [Multiple development branch g.doc](https://docs.google.com/document/d/1LZkxRgzrfVcKttST2d1pNuViPXClb9ssD7Bvt1osRaQ/edit) into a proper Sphinx documentation page.

Objectives of this document:
- explain the dual development branch strategy
- describe the main Jenkins jobs used for testing the current & next major release
- introduce & describe some of the scc utility tools for managing branches & PRs