add new contact page

... to make it easier to find place to report security issues.

content/pages/en/contact.rst

+Contact
+#######
+
+:slug: contact
+:lang: en
+:menu: Contact
+:menuorder: 6
+
+
+* For software support please see `support section <support/>`_.
+
+Email
+=====
+
+* `knot-resolver@labs.nic.cz`_
+
+.. _knot-resolver@labs.nic.cz: mailto:knot-resolver@labs.nic.cz
+
+
+Reporting Security Issues
+=========================
+
+* When reporting sensitive security issues that require encryption, please use
+  both of the following addresses and their associated PGP keys.
+* Our PGP keyring: https://secure.nic.cz/files/knot-resolver/kresd-keyblock.asc
+* `vladimir.cunat@nic.cz`_
+* `tomas.krizek@nic.cz`_
+
+.. _vladimir.cunat@nic.cz: mailto:vladimir.cunat@nic.cz
+.. _tomas.krizek@nic.cz: mailto:tomas.krizek@nic.cz
+
+Address
+=======
+
+* CZ.NIC, z. s. p. o.
+
+  Milesovska 1136/5
+
+  130 00 Praha 3
+
+  Czech Republic