Add to plugins docs: guidance on reports

[davemfish]

Plugin authors who wish to include a reporter module would benefit from some documentation about how to do that.

In particular, because a plugin's jinja2 template will not live in the same directory as natcap.invest.reports.templates, there are a couple steps the developer can take to use/extend those templates that a core model would not need to take.

I have a working branch (still in progress) that demonstrates this here: https://github.com/davemfish/invest-sdr-scenario-compare/tree/task/invest-3-18-reports/src/invest_sdr_scenario_compare