Add document to p4c repo explaining to developers a little bit of what they need to know to add new tests

[jafingerhut]

At the very least, it should explain:

• for "positive" tests, i.e. ones where there is no error from the P4 compiler, what directory should you put the source file in, and what expected output files should be added (and in what directory for those)? Also how to update those if a change to p4c changes the expected outputs.

• for "negative" tests, i.e. the ones where you expect the P4 compiler to catch a syntax or semantic error, what directory should you put the source file in, and what expected output files should be added?

• How to mark a test as "expected to fail" (Xfail), and what does that mean? Note: At least for some existing CI tests, marking a positive test as Xfail means that it must fail, or else the overall CI run will fail. Marking a negative test as Xfail means that it must not give an error, or else the overall CI run will fail. However, because this is a confusing and subtle point, maybe not all p4c CI tests treat negative tests this way. It would be nice to make them all consistent, but at least we should document how they operate now, even if there is a TODO indicating the desire to change it in the future.