docs: use public git URL in the building Rudder section

The README currently recommends cloning the Rudder project using the
SSH git URL, which is not accessible to users outside of the CoreOS
GitHub organization.

Replace the SSH git URL with HTTPS to avoid confusing users and add
clarity to the build documentation.
Merge pull request #36 from kelseyhightower/docs

docs: use public git URL in the building Rudder section