Add a readme explaining how to edit docs.

doc/README

+This directory contains the "source code" for the Cap'n Proto web site.
+
+The site is built with Jekyll.  Start by installing it:
+
+  http://jekyllrb.com/
+
+Be sure to follow the instructions to install Pygments as well.
+
+Next, install the custom Pygments syntax highlighter:
+
+  cd _plugins
+  python capnp_lexer.py install
+  cd ..
+
+Now you can launch a local server:
+
+  jekyll --server --auto --pygments
+
+Edit, test, commit.
+
+If you have permission, after you've pushed your changes back to github, you
+can make your changes live by running:
+
+  ./push-site.sh
+
+Otherwise, send a pull request and let someone else actually push the new
+site.