Add information about Rate Limiting Headers

[workergnome]

Add a section that describes rate limiting, referencing https://tools.ietf.org/id/draft-polli-ratelimit-headers-00.html and other similar specifications.

[tfmorris]

The draft above is expired. The current draft is https://tools.ietf.org/html/draft-polli-ratelimit-headers-03, but probably shouldn't be referenced until it's standardized. The starting point, until then, should probably be to make sure that clients honor Retry-After headers from RFC7231 and mention that service providers may impose rate limits/quotas.

[tfmorris]

Actually, this is a duplicate of #40.

[ioggstream]

It is fair to adopt the ratelimit model without referencing the I-D too :)