Added link to repository, since README also visible on the gh-pages.

README.md

@@ -4,6 +4,9 @@ Standard markdown
 Standard markdown is a [specification of markdown syntax][the spec],
 together with BSD3-licensed implementations (`stmd`) in C and javascript.
 
+The spec and all of the code can be found in
+[this repository](http://github.com/jgm/stmd).
+
 The implementations
 -------------------