CONTRIBUTING: add some initial guidelines

[last-genius]

Some rough guidelines on the contribution process for the project. Intended more as a starting point for a discussion.

[last-genius]

Can be viewed rendered here: https://github.com/last-genius/xen-api/blob/asv/contribution-guide/CONTRIBUTING.md

[lindig]

I believe we should manage expectations by saying that commits that break existing (non-public) tests are likely to be reverted. This may sound unfair because tests are not public but I can't see how we can avoid that. The test infrastructure we have in place is almost impossible to make public, independent of any licensing issues.

[psafont]

The test infrastructure we have in place is almost impossible to make public, independent of any licensing issues.

What could be made public is the operations / features that are considered essential, and are never to be broken. With this, the existing public tests can be enhanced (unit tests or end-to-end xcp-ng ones) to a point where all the tests that are considered for reverting are public, even if it's replicated inside xenserver's testing suite.

[lindig]

We might have public tests not in place immediately; I am in favour to add some words about reverting commits and merge this.

[last-genius]

Added a quick note:

If a commit has been determined to break integration testing at a later stage,
please note that the first and safest measure will almost always be reverting
the faulty commit. Making sure critical tests are passing remains a priority
over waiting for some commit to be reworked or refactored (which can be worked
on after a revert has been done). Though we are striving to make more tests
public (with failure then being visible to all), as long as some critical tests
remain private, this will also apply to such tests (with maintainers flagging
the breakage preferrably describing at least the gist of the test).

[psafont]

I can't think of anything in particular that the document is missing. I'd like Rob to review it, however