Change project description to clarify contribution process

Project description:

Want to contribute?

Due to issues with the automatic testing Drupal.org provides, we are unable to run the tests created for the module. Because of this, we have changed the way we collaborate around commerce shipping. Instead of making patches, we would like to receive contributes as pull requests on the official mirror on github. Patches are still welcome, but it's a big help if you can make a PR on github instead.

Per Bojanz:

Let's not use GitHub for PRs, much bigger chance of code being forgotten that way. Upload a patch to the d.o issue.