Document the plans to compile "The 55" in the README

[masak]

The name "The 55" is a name I've started using increasingly. These are the initial 55 definitions in bel.bel, before the evaluator:

• One global value (vmark)
• 15 macros
• 39 functions

Once we can compile these well, we can start thinking about compiling the bel evaluator, with all its functions and massively tail-recursive structure. (There are self-calling functions among the 55, but no mutually calling functions.)

Might also be interesting at that point to provide a REPL of sorts for the fast subset of Bel that includes The 55.

[masak]

This issue could help finally merge #216.