[PATCH 4/4] Add docs for notary mysql

This patch add the recommendation to guide people deploying a more
secure MySQL for notary.

Signed-off-by: Hu Keping <hukeping@huawei.com>

docs/notary-mysql.md

@@ -0,0 +1,23 @@
+<!--[metadata]>
++++
+title = "Notary MySQL"
+description = "Description of the Notary MySQL"
+keywords = ["docker, notary, notary-mysql"]
+[menu.main]
+parent="mn_notary"
++++
+<![end-metadata]-->
+
+# Notary MySQL
+
+The Notary MySQL is one of the backends for [Notary Server](notary-server.md) and
+[Notary Signer](notary-signer.md).
+
+### Recommendation
+For security, especially in production deployments, one should create users
+with restricted permissions and separate databases for the `server` and
+`signer` since the `signer` only needs the `private_keys` table, and the
+`server` only needs `timestamp_keys` and `tuf_files`.
+
+We use such a setup in our compose file to provide people with more accurate
+guidance in deploying their own instances.