Add simple build instructions to the README.

Author: gkellogg

README.md

@@ -1,4 +1,6 @@
 # rdf-tests
 Repository for the RDF Tests Community Group
 
-See the [gh-pages](https://w3c.github.io/rdf-tests) branch for the main contents.
+See the [gh-pages](https://w3c.github.io/rdf-tests) branch for the main contents.
+
+The index.html files are built using a Ruby tool-chain based on the corresponding manifest.ttl files. Update using `rake`. Requires Ruby, rdf-turtle and json-ld gems to be installed.