Improve documentation/integration of yetus about eve

[zach-welch-timesys]

With #306 merged, the first item on my list has been completed.

[aw-was-here]

3. The existing tooling does not appear to produce a report comparable to the output produced by the CricleCI integration on GitHub. Notably, I have only been able to produce a full report containing all of the issues in the tree, when I expect a differential report that contains only the issues introduced by the changes that I have made.

FWIW, this is half limitation of Apache Yetus and half limitation in how the Makefile has been structured.

Low-level details:

test-patch supports two modes of operation, a comparison against another branch and full build testing. The difference in when it does which is dependent upon what information the software can glean from the environment and what flags are passed to it. Under Circle CI, lots of additional context is provided either by Circle CI itself or from querying the GitHub API by using some of the Circle CI information. When one runs 'make yetus' locally, all of that context is missing. Another part of the puzzle is that when test-patch detects it is running under some sort of automation ('robots' in Apache Yetus parlance), it knows it can run in a destructive mode. That means switch the branch, destroy uncommitted/excess files, etc. Needless to say, on a local setup, that may be highly surprising.

It might be possible to add a target to the Makefile that provides the necessary context and lets test-patch run in the destructive mode to provide the diff-like capabilities. Off the top, I don't think it'd be super hard, but it would definitely be tricky. I'm not sure how much make likes the Makefile changing out from underneath it! :)