Restructure README

[mpadge]

There's currently just too much information there for it to be readily usable. I suggest we need to find an alternative home for a lot of that stuff. The raw yaml chunks in particular should go in a separate document on "Workflow Customisation" or the likes. The question is where to put this? Thoughts:

1. In a sub-folder as straight-up markdown so GitHub will render it directly
2. Switch on GitHub pages here and render -> serve the doc that way

(There are other options, but I think these are the best.) The first is probably easier, and will also integrate more directly with https://ropensci-review-tools.readthedocs.io. We can then link directly to those docs from the main page. Thoughts @assignUser?

[mpadge]

The above link demonstrates one need for the docs to more clearly explain and demonstrate typical use cases. In that case, adding a simple if condition to only run the checks when commit messages contain "pkgcheck".