Put developer guidance in the repository #386

I notice that the CPython repository now contains information on the implementation, migrated from the Dev Guide. Let's migrate ours to here. I think it is more likely to be kept up to date if it is in close proximity to the code.

I find this kind of information about CPython helpful. I find it helpful to write similar things about Jython too, and believe it helps others to read them.

Java affords us better ways than C to document implementation in the source itself. However, there is logic that spans classes and packages, for which some sort of definitive guide would have been helpful to me as a beginner ... more honestly, I could do with it now.

We have bits of that kind of information in the old wiki, where it feels constantly at risk, and material in a few other scattered locations.

The CPython Dev Guide seems likely to retain its sections on contribution and release process. This makes sense because it applies to more than one repository (including ours). Our Dev Guide tries to piggy-back off the CPython one, but it's too complicated to maintain. It could be more manageable if it didn't pretend to be the architecture guide too.