update CONTRIBUTING.rst

[paulthebaker]

There is some confusion as to what is within the scope of enterprise, what is better suited to enterprise_extensions, and what is best kept in totally separate code in one's own project.

I think enterprise should be limited to the essential building blocks common to nearly all analyses. The pitch is "enterprise makes a likelihood, you do what you want with that." I'm not sure we've always kept to this philosophy ourselves.

We should update the CONTRIBUTING.rst document to give clear guidance as to what is and isn't suitable for enterprise. This means we need to figure out what those guidelines are and possibly retroactively apply them to existing code.

[jellis18]

Related to #255