Create CONTRIBUTING.md file

[waldyrious]

• Add CONTRIBUTING.md file
• Format markdown files to break long lines and improve wording

For future reference, this PR was prompted by this discussion in #133.

[Xymph]

No remarks on contributing/governance/changelog.

In Usage, consider linefeeds at 58, 87, 185, 311;
and in Readme at 10-11, 19-20 and more (looks like you skipped it entirely).

[waldyrious]

Great suggestions for USAGE.md. I've applied them all. Same in the README. I hadn't really looked into lines that might need re-wrapping; I only handled lines that were already relatively long. I have now tried to locate other instances where re-wrapping might make sense.

As for lines 10-11 in the README, I'm not sure what wrapping would work better in your view (without breaking the sentence's clauses apart). I've moved the second sentence to its own line, but I can't think of a better way to wrap the first sentence. If anything, I could make it longer, by moving the "is v0.15.0," to the first line. Is this what you had in mind?

[Xymph]

Almost there! 😃

[Xymph]

And that's a wrap! 👍