Thanks for your interest in contributing to devtools-snippets!
If you'd like to add a new snippet: Take a look at an existing snippet: https://github.com/bgrins/devtools-snippets/tree/master/snippets/jquerify. You can copy this format into /snippets/your-snippet/ with the README.md documenting and linking to your screenshot. The README will be copied into the home page at http://bgrins.github.io/devtools-snippets/
If you'd like to modify an existing snippet: Feel free to make improvements to an existing snippet. Do not worry about rebuilding index.html, just make sure the README looks good and I will rebuild the site before pushing up changes.
If you'd like to modify the HTML: Be sure to change template.html, not index.html (it is autogenerated). By running rake build, you can preview changes. No need to include the autogenerated file in a pull request, I will rebuild it before pushing up changes.