Improve Contributing section

Author: CrazyPython

README.md

@@ -14,3 +14,5 @@ The redness represents how active an API's proposal or specification is.
 * Element abbreviation links to What can this do? (Demo, Playground, or Reference)
 * Details links to How can I learn this? Guide
 * Background links to browser support (For most APIs, caniuse.com. For in-progress APIs, implementation progress pages.)
+## Contributing
+Simply edit the HTML file. :)  It uses nicely formatted semantic HTML. The CSS is inside the file.