Add dkim module documentation.

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+# DKIM module
+
+This module checks [DKIM](http://www.dkim.org/) signatures for emails scanned.
+DKIM signatures can establish that this specific message has been signed by a trusted
+relay. For example, if a message comes from `gmail.com` then a valid DKIM signature
+means that this message was definitely signed by `gmail.com` (unless gmail.com private
+key has been compromised, which is not a likewise case).
+
+## Principles of work
+
+Rspamd can deal with many types of DKIM signatures and messages canonicalisation.
+The major difficulty with DKIM are line endings: many MTA treat them differently which
+leads to broken signatures. Basically, rspamd treats all line endings as `CR+LF` that
+is compatible with the most of DKIM implementations.
+
+## Configuration
+
+DKIM module has several useful configuration options:
+
+- `dkim_cache_size` (or `expire`) - maximum size of DKIM keys cache
+- `whitelist` - a map of domains that should not be checked with DKIM (e.g. if that domains have totally broken DKIM signer)
+- `domains` - a map of domains that should have more strict scores for DKIM violation
+- `strict_multiplier` - multiply the value of symbols by this value if received from `domains` map
+- `trusted_only` - do not check DKIM signatures for all domains but those which are from the `domains` map
+- `skip_multi` - skip DKIM check for messages with multiple signatures
+
+The last option can help for some circumstances when rspamd lacks the proper support of
+multiple DKIM signatures. Unfortunately, with some mailing lists, or other software
+this option could be useful to reduce false positives rate as rspamd deals with
+multiple signatures poorly: it just uses the first one to check. On the other hand,
+the proper support of multiple DKIM signatures is planned to be implemented in rspamd 
+in the next releases, which will make this option meaningless.
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