Document contribution process

[MarkMcCaskey]

The issue came up on spectrum that we don't document how to run tests, for example, or how to deal with our release process in general.

We should create a small document and link to it in visible places (possibly by updating the issue filing template) that explains how bors works, links to bors's documentation, and gives information on the rest of our process.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[Hywan]

Closing since we have https://github.com/wasmerio/wasmer/blob/master/CONTRIBUTING.md, introduced in #684 and updated later.