doco glossary: Explain some terms

[cben]

A couple months ago I wanted to start understanding the implementation, and decided the evaluator might be a portable and self-contained place to start. My reading time being mostly offline, I'd printed out some docs/ files, some definitions, macros, stacks.lisp, and eval.lisp. Well, that proved naive :rofl:

• The current docs/*.md files are insufficient to a newcomer.
• There terminology is in places archaic and is a barrier to entry.
• The system design is unusually integrated, compared to most langagues. This is pretty essential to the naive realism goals, but makes it harder to find an "isolated" place to start. Newcomers could really use a "map".
• The implementation is much more optimized (less "naive") than I thought.

After a break for reading Changing Minds, I also found Leigh Klotz's thesis and that was a godsend! After that I managed to grok stacks.lisp, and follow the general structure of eval.lisp, even if not all is clear (duh!).

I don't know how much time I'll have to contribute, so I figured "smoothing the on-ramp" is a useful area for me to start => here is first step, adding a README link to that thesis + filling out some of the glossary.
I'm sure there are inaccuracies, please review!

[cben]

P.S. when writing some of the explanations, I found it hard to concisely explain general principles, and easier to follow a specific example. E.g. repeat is a great example for a recursive-eval primitive — it's simple enough but shows the need to keep state (loop counter) and the ability of the :after part to do decide between poping the frame vs doing another iteration.

If I could dream, I'd want a "behind the scenes stepper" that runs the evaluator [including in the non-stepping optimized mode!] and traces the internal state. Well, are there any unit tests actually running the evaluator?

[cben]

From my side this PR is stable and ready for review.

[sgithens]

Thanks @cben ! I think this is probably good, sorry I haven't read through it more, thanks @amb26 for reviewing it. I'll try to merge this in the day couple days.