Add a simple README in the doc folder for guidance

doc/README.md

@@ -0,0 +1,10 @@
+Cowboy documentation
+====================
+
+Documentation for Cowboy is available online at the following addresses:
+
+ *  [README](http://ninenines.eu/docs/en/cowboy/HEAD/README)
+ *  [User Guide](http://ninenines.eu/docs/en/cowboy/HEAD/guide/introduction)
+
+This folder is used for generating the Reference Manual. You can generate
+it yourself by typing `make docs` in the top-level Cowboy directory.