Add documentation for configuring rate limits

I have a merge-prop, https://code.launchpad.net/~klmitch/nova/lp759978/+merge/67881, for nova that adds the ability to configure rate limiting using the api-paste.ini file. Brian Waldon suggested that I should open a ticket over here to have this documented. Quick run-down:

In the [filter:ratelimit] section of api-paste.ini, the following new relations are available:

* limiter - Allows specification of an alternate Limiter class. The de-facto default value is "nova.api.openstack.limits.Limiter"

* limits - Allows specification of default rate limit rules. See below for syntax.

* user:<username> - Allows specification of rate limit rules per username. See below for syntax.

Rate limit rules are a sequence of semicolon-separated, parenthesized groups of 5 arguments, where the arguments are separated by commas. The 5 arguments are:

* Verb - The HTTP verb, i.e., 'GET'. Values are coerced to upper-case.

* URI - The human-readable URI. Used only for describing the rule to users.

* Regex - A regular expression for matching the URI.

* Value - An integer specifying the number of accesses permitted per unit of time.

* Unit - One of "second, minute, hour, day" (case-insensitive) specifying the unit of time for interpreting the value.

The default rate limit rules can be expressed as follows: (POST, *, .*, 10, minute); (POST, */servers, ^/servers, 50, day); (PUT, *, .*, 10, minute); (GET, *changes-since*, .*changes-since.*, 3, minute); (DELETE, *, .*, 100, minute)

(Note that rate limit rule parsing and per-user limits can be affected by specifying a different limiter; what I've described is just the default.)