feature(docbook): switch to docbook 5.1 and relaxng validation

see #262

Author: duncdrum

README.md

@@ -1,6 +1,6 @@
 # eXist-db Documentation
 [![Build Status](https://travis-ci.com/eXist-db/documentation.svg?branch=master)](https://travis-ci.com/eXist-db/documentation)
-[![Docbook version](https://img.shields.io/badge/docbook-5.0-19a5a4.svg)](http://docbook.org/xml/5.0/)
+[![Docbook version](https://img.shields.io/badge/docbook-5.1-19a5a4.svg)](http://docbook.org/xml/5.1/)
 [![eXist-db version](https://img.shields.io/badge/eXist_db-5.2.0-blue.svg)](http://www.exist-db.org/exist/apps/homepage/index.html)
 
 <img src="src/main/xar-resources/icon.png" align="left" width="15%"/>