feat(all): add a LLM instruction file

Summary: This diff adds a machine-readable, repo-native document that teaches public LLMs (in case they have the ability to visit github repo or even auto crawl the md files) on how MemLab is intended to be used, what principles to follow, and what mistakes to avoid when generating code or explanations.

Differential Revision: D90453129

fbshipit-source-id: f7e5ebe212a8ac7ce44a22c649f59eea6aa01ed4

Author: JacksonGL