update readme

README.md

@@ -2,4 +2,25 @@
 
 [![Build Status](https://jenkins.testbed.tier.internet2.edu/buildStatus/icon?job=docker/shib-sp/master)](https://jenkins.testbed.tier.internet2.edu/job/docker/shib-sp/master)
 
+This is the TIER upstream Shibboleth SP container.
+
+It is based from CentOS 7 and includes httpd, mod_ssl, and the current shibboleth SP.
+
+Files you must supply/override in your downstream builds:
+
+The SP's private key and corresponding certificate (very important!) can be generated in your downstream container like this:
+     RUN /usr/local/bin/shibboleth_keygen.sh -o /etc/shibboleth -f
+           ...that command generates/overwrites the following files:
+                       /etc/shibboleth/sp-key.pem
+                       /etc/shibboleth/sp-cert.pem
+
+/etc/httpd/conf.d/ssl.conf
+   including:
+      ServerName fqdn:port
+      UseCanonicalName On
+
+/etc/shibboleth/shibboleth2.xml
+   including:
+      entityID
+