-- Please review for accuracy prior to merge --

While I'm still learning the external pillar system, I suspect that this documentation was written when a smaller number of external pillars were available. I have removed the plain-text reference and instead linked directly to the repository directory which holds all pillars. If my assumption/understanding is incorrect, please feel free to reject. :]
The documentation for 'Developing Salt' is missing a critical step for 0.17+ users who wish to utilize the test runner. This commit describes the direct installation of salt-testing.

Forthcoming will be a re-write what appears to be a new method for establishing a development environment through the use of dev_requirements_python2(6|7).txt, once I have a chance to verify
that the workflow is as expected.