Add some build instructions for how to install doc dependencies.

jq can now build without Ruby, but you won't get a nice manpage.

docs/default_manpage.md

@@ -0,0 +1,22 @@
+jq(1) -- Command-line JSON processor
+====================================
+
+## DESCRIPTION
+
+`jq` can transform JSON in various ways, by selecting, iterating,
+reducing and otherwise mangling JSON documents. 
+
+This version of `jq` was built without a manual, so this manpage is a
+stub. For full documentation of the `jq` language, see:
+
+    http://stedolan.github.com/jq
+    
+## BUGS
+
+Presumably. Report them or discuss them at:
+
+    https://github.com/stedolan/jq/issues
+
+## AUTHOR
+
+Stephen Dolan `<mu@netsoc.tcd.ie>`