Create OP_CODEGEN_GUIDE.md (#3687)
* Create OP_CODEGEN_GUIDE.md
Migrate the internal codegen guide by @JackCaoG to README. Re-worded and re-structured the README so it can be shared publicly.
* Fix some typos
* Update some wordings
* Update instructions to build pt/xla
* Rename the readme file and add steps to run C++ tests
