[Cookbook][Workflow] Mention alternatives to GitHub

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -

I think it would be fair to mention alternatives to GitHub. As Bitbucket offers free private repos and is also quite popular, I mentioned it explicitly. In addition, there's a link to a comparision of web-hosting services for VCS.

Do not hesitate to fix my English...