New issue template, and contributing guidelines

[jfbastien]

I think ~ now is a good time to update our contributing guidelines, and maybe adopt a new issue template:

• We already have a simple Contributing.md, should we improve it?
• All other repos should point to it, is that the case? (it was when I first set this up)
• I think we should add a new issue template. This repo's should be helpful, and the one in other repos should link to the design repo (except maybe spec repo, which should disambiguate spec and design issue).

Thoughts?

@s3ththompson, could you tackle this?

[s3ththompson]

Will do this afternoon.

[s3ththompson]

Ran out of time, will put this on my todo for tomorrow.

[jfbastien]

@s3ththompson ping?