document that valve is thing included in releases

README.md

-# MyUW root webapp
+# MyUW root webapp and Tomcat error valve
 
-This is the root context of MyUW. It is just here to serve up some static html
-pages for 500 and 400 level errors.
+There are two modules here: a webapp to serve as the root web application
+context and plugin to customize Tomcat's error handling.
 
-## Development
+## MyUW root webapp
+
+This is a small webapp to serve as the root, default servlet application in MyUW
+Tomcat. It contains the error pages to fall back upon when another servlet
+context does not handle the request.
+
+### Development of the root webapp
 
 From the root directory, run `npm start`. The web app will run at
 [http://localhost:8080](http://localhost:8080) and will show the 404 page by
 default.
+
+## MyUW Tomcat error valve
+
+Customizes Tomcat to use the error pages in the custom root webapp.
+
+## Releasing
+
+The root webapp and the valve are versioned and released together.
+
+From the root directory of this project
+
+```
+mvn release:prepare
+mvn release:perform
+```
+
+When you release a new version of this project, you'll publish both a root
+webapp .war and a Tomcat error valve .jar.
+
+For these to be actually used in MyUW, update the tier-specific ears to
+[reference the new root webapp version][MyUW ear use of root webapp],
+and update the Tomcat container to
+[use the new valve version][MyUW Tomcat use of valve].
+
+[MyUW ear use of root webapp]: https://git.doit.wisc.edu/myuw-overlay/myuw-ear/-/blob/qa/pom.xml#L54
+[MyUW Tomcat use of valve]: https://git.doit.wisc.edu/myuw/tomcat7-java8/-/blob/master/Dockerfile#L8
+