create contributions.md describing how to contribute to dynare

I don't know about the unitary tests, so @stepan-a can contribute that part

@houtanb For the reporting bugs-part, could you please add that people please only report bugs for the unstable or the most recent stable version, might want to check against http://www.dynare.org/DynareWiki/KnownBugs and that they should please clearly state:

• Dynare version used
• Octave/Matlab version used

@JohannesPfeifer this is a good idea, though I think people should be able to report bugs in both stable and unstable. I'll add it now.

Already done in CONTRIBUTING.md.

closed

Oh sorry the unitary tests part is missing, reopening.

reopened

removed milestone

changed milestone to %4.6

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mentioned in commit fec27413

changed milestone to %5.x

As example for the syntax of unitary tests is dseries@be4a4d39

mentioned in merge request !1912 (merged)

closed with commit 97a86b5b