Document the standard Rate Limit headers that the SDK implements

[rhamzeh]

Currently the SDKs expect:

• X-RateLimit-Limit: The maximum number of requests in a time period.
• X-RateLimit-Remaining: The number of requests remaining in the current rate limit window.
• X-RateLimit-Reset: the time at which the current rate limit window resets in UTC epoch seconds"

We're also considering support for Retry-After