Add deployment notes

DEPLOYMENT-NOTES.rst

+Deployment notes
+================
+
+- 2021-09-07 `Manage access to payment metrics <https://whetstone.privatestorage.io/privatestorage/PrivateStorageio/-/merge_requests/146>`_ requires moving and chown'ing the PaymentServer database on the ``payments`` host::
+
+   mkdir /var/lib/zkapissuer
+
+   mv /var/db/vouchers.sqlite3 /var/lib/zkapissuer/vouchers.sqlite3
+
+   chown -R zkapissuer:zkapissuer /var/lib/zkapissuer
+
+   chmod 750 /var/lib/zkapissuer
+   chmod 640 /var/lib/zkapissuer/vouchers.sqlite3
+