Add SPF module documentation.

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+# SPF module
+
+SPF module performs checks of the sender's [SPF](http://www.openspf.org/) policy.
+Many mail providers uses SPF records to define which hosts are eligible to send email
+for this specific domain. In fact, there are many possibilities to create and use
+SPF records, however, all they check merely the sender's domain and the sender's IP.
+
+The specific case are automated messages from the special mailer daemon address:
+`<>`. In this case rspamd uses `HELO` to grab domain information as specified in the
+standart.
+
+## Principles of work
+
+`SPF` can be a powerfull tool when properly used. However, it is very fragile in many
+cases: when a message is somehow redirected or reconstructed by mailing lists software.
+
+Moreover, many mail providers have no clear understanding of this technology and
+misuse the SPF technique. Hence, the scores for SPF symbols are relatively small
+in rspamd.
+
+SPF uses DNS service extensively, therefore rspamd maintain the cache of SPF records.
+This caches operates on principle of `least recently used` expiration. All cached items
+lifetimes is accordingly limited by the matching DNS record time to live.
+
+You can manually specify the size of this cache by configuring SPF module:
+
+~~~nginx
+spf {
+	spf_cache_size = 1k; # cache up to 1000 of the most recent SPF records
+}
+~~~
+
+Currently, rspamd supports the full set of SPF elements, macroes and has internal
+protection from DNS recursion.
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