[spec-reviews] contributing.md document needed for tag reviews (#53)

Document the following:

* timeliness of required review
* has a security review been done?
* expectation of what happens next

---
Reply to this email directly or view it on GitHub:
https://github.com/w3ctag/spec-reviews/issues/53

Received on Thursday, 23 April 2015 21:59:06 UTC