Add documentation fro ratelimit module.

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+# Ratelimit plugin
+
+Ratelimit plugin is designed to limit messages coming from certain senders, to
+certain recipients from certain IP addresses combining these parameters into
+a separate limits.
+
+## Principles of work
+
+The basic principle of ratelimiting in rspamd is called `leaked bucket`. It could
+be visually represented as a bucket that has some capacity, and a small hole in a bottom.
+Messages comes to this bucket and leak through the hole over time (it doesn't delay messages, just count them). If the capacity of
+a bucket is exhausted, then a temporary reject is sent. This happens unless the capacity
+of bucket is enough to accept more messages (and since messages are leaking then after some
+time, it will be possible to process new messages).
+
+Rspamd uses 3 types of limit buckets:
+
+- `to` - a bucket based on a recipient only
+- `to:ip` - a bucket combining a recipient and a sender's IP 
+- `to:from:ip` - a bucket combining a recipient, a sender and a sender's IP
+
+For bounce messages there are special buckets that lack `from` component and have more
+restricted limits. Rspamd treats the following senders as bounce senders:
+
+- 'postmaster',
+- 'mailer-daemon'
+- '' (empty sender)
+- 'null'
+- 'fetchmail-daemon'
+- 'mdaemon'
+
+Each recipient has its own triple of buckets, hence it is useful
+to limit number of recipients to check.
+
+Each bucket has two parameters:
+- `capacity` - how many messages could go into a bucket before a limit is reached
+- `leak` - how many messages per second are leaked from a bucket.
+
+For example, a bucket with capacity `100` and leak `1` can accept up to 100 messages but then
+will accept not more than a message per second.
+
+By default, ratelimit module has the following settings:
+
+~~~lua
+-- Default settings for limits, 1-st member is burst, second is rate and the third is numeric type
+local settings = {
+  -- Limit for all mail per recipient (burst 100, rate 2 per minute)
+  to = {[1] = 100, [2] = 0.033333333, [3] = 1},
+  -- Limit for all mail per one source ip (burst 30, rate 1.5 per minute)
+  to_ip = {[1] = 30, [2] = 0.025, [3] = 2},
+  -- Limit for all mail per one source ip and from address (burst 20, rate 1 per minute)
+  to_ip_from = {[1] = 20, [2] = 0.01666666667, [3] = 3},
+
+  -- Limit for all bounce mail (burst 10, rate 2 per hour)
+  bounce_to = {[1] = 10, [2] = 0.000555556, [3] = 4},
+  -- Limit for bounce mail per one source ip (burst 5, rate 1 per hour)
+  bounce_to_ip = {[1] = 5 , [2] = 0.000277778, [3] = 5},
+
+  -- Limit for all mail per user (authuser) (burst 20, rate 1 per minute)
+  user = {[1] = 20, [2] = 0.01666666667, [3] = 6}
+
+}
+~~~
+
+All limits are stored in [redis](http://redis.io) server (or servers cluster).
+
+## Module configuration
+
+`Ratelimit` module can be configured to setup the following:
+
+- `whitelisted_rcpts` - comma separated list of whitelisted recipients. By default
+the value of this option is 'postmaster, mailer-daemon'
+- `whitelisted_ip` - a map of ip addresses or networks whitelisted
+- `max_rcpts` - do not apply ratelimit if it contains more than this value of recipients (5 by default). This
+option allows to avoid too many work for setting buckets if there are a lot of recipients in a message).
+- `limit` - allows to set limit for a specific category. This option should be in the following form:
+
+    type:burst:leak
+
+Where `type` is one of:
+
+- `to`
+- `to_ip`
+- `to_ip_from`
+- `bounce_to`
+- `bounce_to_ip`
+
+`burst` is a capacity of a bucket and `leak` is a rate in messages per second.
+Both these attributes are floating point values.
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