Add documentation for RBL module

doc/markdown/modules/rbl.md

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+# RBL module
+
+The RBL module provides support for checking the IPv4/IPv6 source address of a message's sender against a set of RBLs as well as various less conventional methods of using RBLs: against addresses in Received headers; against the reverse DNS name of the sender and against the parameter used for HELO/EHLO at SMTP time.
+
+Configuration is structured as follows:
+
+~~~nginx
+rbl {
+  # default settings
+  an_rbl {
+    # rbl-specific subsection 
+  }
+  # ...
+}
+~~~
+
+The default settings define the ways in which the RBLs are used unless overridden in an RBL-specific subsection.
+
+Defaults may be set for the following parameters (default values used if these are not set are shown in brackets):
+
+- default_ipv4 (true)
+
+Use this RBL to test IPv4 addresses.
+
+- default_ipv6 (false)
+
+Use this RBL to test IPv6 addresses.
+
+- default_received (false)
+
+Use this RBL to test IPv4/IPv6 addresses found in Received headers. The RBL should also be configured to check one/both of IPv4/IPv6 addresses.
+
+- default_from (true)
+
+Use this RBL to test IPv4/IPv6 addresses of message senders. The RBL should also be configured to check one/both of IPv4/IPv6 addresses.
+
+- default_rdns (false)
+
+Use this RBL to test reverse DNS names of message senders (hostnames passed to rspamd should have been validated with a forward lookup, particularly if this is to be used to provide whitelisting).
+
+- default_helo (false)
+
+Use this RBL to test parameters sent for HELO/EHLO at SMTP time.
+
+- default_unknown (false)
+
+If set to false, do not yield a result unless the response received from the RBL is defined in its related returncodes {} subsection, else return the default symbol for the RBL.
+
+RBL-specific subsection is structured as follows:
+
+~~~nginx
+# Descriptive name of RBL or symbol if symbol is not defined.
+an_rbl {
+        # Explicitly defined symbol
+	symbol = "SOME_SYMBOL";
+        # RBL-specific defaults (where different from global defaults)
+        #The global defaults may be overridden using 'helo' to override 'default_helo' and so on.
+        ipv6 = true;
+	ipv4 = false;
+	# Address used for RBL-testing
+	rbl = "v6bl.example.net";
+	# Possible responses from RBL and symbols to yield
+	returncodes {
+		# Name_of_symbol = "address";
+		EXAMPLE_ONE = "127.0.0.1";
+		EXAMPLE_TWO = "127.0.0.2";
+       }
+}
+~~~
+