Add examples showing how to apply rate limiting

[catbrained]

Rate limiting seems like a common use case, so it might be useful to add some simple examples that demonstrate how to apply it to a client.

Some scenarios:

• How to apply a global rate limit to an entire client
• How to apply a rate limit to connection establishment using connector_layer()
• How to apply different rate limits to different routes or hosts

(The last one might be a more involved example.)