Doc running pants from sources in other repos.
Doc running pants from sources in other repos.

This documents the non-obvious way pants needs to be run when plugins
are involved.

Testing Done:
Rendered here:
  https://github.com/jsirois/pants/blob/jsirois/sneakernet/document/src/python/pants/docs/howto_develop.md#running-from-sources
  http://pantsbuild.github.io/staging/jsirois/howto_develop.html#running-from-sources

CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/123980690

Bugs closed: 3219

Reviewed at https://rbcommons.com/s/twitter/r/3715/

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