README should explain usage

[scottj97]

The current README.md does not explain how one would use this repo to add support for new instructions. I assume this repo can somehow be used to generate files useful to Spike and/or binutils?

There is some evidence for this in Spike's git repo:

riscv/encoding.h:/* Automatically generated by parse_opcodes.  */

Currently the README only refers to riscv-tools, which is (1) obsolete, and (2) doesn't seem to actually use this repo for anything. (At least, I couldn't find any script that refers to its riscv-opcodes submodule, or the parse_opcodes script.)

[furuame]

You could check out the Makefile.