add document covering pros and cons of creating a plugin versus contributing to an existing one

[gregcaporaso]

Addition Description
It would be helpful to have a document that describes pros and cons of building a new plugin versus contributing to an existing (possibly "core distribution") plugin.

References
@thermokarst and I started discussing this in the context of this PR.

[gregcaporaso]

I have content on this in the new CONTRIBUTING.md doc.