Document more clearly how to test alternate operator builds

Improve the developer documentation such that it is very clear how to test alternate builds of the operator for testing and reviews.

The following discussion from !155 (merged) should be addressed:

• @rmarshall started a discussion: (+3 comments)

  A note for something I ran into that can be resolved to another issue:

  when testing, this make install_crds does not take into account the git branch that is checked out.

  I had to manually edit the operator CRD to point at the proper tagged image.