Various trivial improvements to README, add CONTRIBUTING info, and re-gen SSL example cert/key

* Improvements to README files
* Add CONTRIBUTING.md
* Regenerate example SSL certificate/key using modified commands
* Update canonical location of several submodules that have moved
* Bump lua-nginx-module for doc fixes (openresty/lua-nginx-module@7a20b0770573343c73b8397487e8f04548c17c83)