Add multimap documentation.

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+# Multimap module
+
+Multimap module is designed to handle rules that are based on different types of maps.
+
+## Principles of work
+
+Maps in rspamd are the files or HTTP links that are automatically monitored and reloaded
+if changed. For example, maps can be defined as following:
+
+	"http://example.com/file"
+	"file:///etc/rspamd/file.map"
+	"/etc/rspamd/file.map"
+
+Rspamd respects `304 Not Modified` reply from HTTP server allowing to save traffic
+when a map has not been actually changed since last load. For file maps, rspamd uses normal
+`mtime` attribute (time modified). The global map watching settings are defined in the
+`options` section of the configuration file:
+
+* `map_watch_interval`: defines time when all maps are rescanned; the actual check interval is jittered to avoid simultaneous checking (hence, the real interval is from this value up to the this interval doubled).
+
+Multimap module allows to build rules based on the dynamic maps content. Rspamd supports the following
+map types in this module:
+
+* `hash map` - a list of domains or `user@domain`
+* `ip map` - an effective radix trie of `ip/mask` values (supports both IPv4 and IPv6 addresses)
+* `cdb` - constant database format (files only)
+
+Multimap has different message attributes to be checked via maps.
+
+## Configuration
+
+The module itself contains a set of rules in form:
+
+	symbol { type = type; map = uri; [optional params] }
+
+Type attribute means what is matched with this map. The following types are supported:
+
+* `ip` - matches source IP of message (radix map)
+* `from` - matches envelope from (or header `From` if envelope from is absent)
+* `rcpt` - matches any of envelope rcpt or header `To` if envelope info is missing
+* `header` - matches any header specified (must have `header = "Header-Name"` configuration attribute)
+* `dnsbl` - matches source IP against some DNS blacklist (see [RBL](rbl.md) documentation for details)
+
+DNS maps has historic support.
+
+Maps can also have a special URL format in style:
+	
+	map = "cdb:///path/to/file.cdb";
+
+which is treated as CDB map by rspamd.
+
+Here is an example configuration of multimap module:
+
+~~~nginx
+multimap {
+	test { type = "ip"; map = "/tmp/ip.map"; symbol = "TESTMAP"; }
+	spamhaus { type = "dnsbl"; map = "pbl.spamhaus.org"; symbol = "R_IP_PBL"; 
+		description = "PBL dns block list"; } # Better use RBL module instead
+}
+~~~
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