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/>`_.
+* To report security issues please e-mail us and encrypt the message using keys below.
+
+Email
+=====
+
+* `knot-resolver@labs.nic.cz`_
+
+.. _knot-resolver@labs.nic.cz: mailto:knot-resolver@labs.nic.cz
+
+GPG keys for secure communication
+---------------------------------
+
+- `4A8BA48C2AED933BD495C509A1FBA5F7EF8C4869.asc
+  <https://keys.openpgp.org/vks/v1/by-fingerprint/4A8BA48C2AED933BD495C509A1FBA5F7EF8C4869>`__
+- `B6006460B60A80E782062449E747DF1F9575A3AA.asc
+  <https://keys.openpgp.org/vks/v1/by-fingerprint/B6006460B60A80E782062449E747DF1F9575A3AA>`__
+
+Address
+=======
+
+* CZ.NIC, z. s. p. o.
+
+  Milesovska 1136/5
+
+  130 00 Praha 3
+
+  Czech Republic