add a simple guide for notary on harbor

docs/use_notary.md

@@ -0,0 +1,23 @@
+### Setup
+In harbor.cfg, make sure the attribute ```ui_url_protocol``` is set to ```https```, and the attributes ```ssl_cert``` and ```ssl_cert_key``` are pointed to valid certificates.  For more information about generating https certificate please refer to:  [Configuring HTTPS for Harbor](configure_https.md) 
+
+### Copy Root Certificate
+Suppose the Harbor instance is hosted on a machine ```192.168.0.5```
+If you are using a self-signed cetificate, make sure to copy the CA root cert to ```/etc/docker/certs.d/192.168.0.5/``` and ```~/.docker/tls/192.168.0.5/```
+
+### Enable Docker Content Trust
+It can be done via setting envrironment variables:
+
+```
+export DOCKER_CONTENT_TRUST=1
+export DOCKER_CONTENT_TRUST_SERVER=https://192.168.0.5/notary
+```
+
+### Set alias for notary (optional)
+Because by default the local directory for storing meta files for notary client is different from docker client.  If you want to use notary client to manipulate the keys/meta files generated by Docker Content Trust, please set the alias to reduce the effort:
+
+```
+alias notary="notary -s https//192.168.0.5 -d ~/.docker/trust --tlscacert  /
+etc/docker/certs.d/192.168.0.5/ca.crt"
+
+```