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/README.md

@@ -0,0 +1,25 @@
+Documentation
+=============
+
+The jq website, manpages and some of the tests are generated from this
+directory. The directory holds a [Bonsai](http://tinytree.info)
+website, and the manual is a YAML file in `content/3.manual`.
+
+To build the documentation (including building the jq manpage), you'll
+need a working Ruby setup. The easiest way to get one is to install
+RVM and Ruby 1.9.3 like so:
+
+    \curl -L https://get.rvm.io | bash -s stable --ruby=1.9.3
+
+After that finishes installing, you'll need to make sure RVM is on
+your path by doing `source $HOME/.rvm/scripts/rvm`, or just opening a
+new shell. See <http://rvm.io> for more info on RVM.
+
+Once RVM is installed, you can install all the dependencies for jq's
+documentation build by running this from the `docs` directory:
+
+    bundle install
+
+When bundle manages to install the dependencies, rerun `./configure`
+in the jq root directory and then the Makefile will be able to
+generate the jq manpage.