Add `CONTRIBUTING.md`

[KFAFSP]

For reference, click here.

LLVM guidelines, but:

• We ship our own clang format style
• We require doxygen in our style
• We require invariant pre/post condition documentation everwhere
• We prefer std types over LLVM/MLIR everywhere, even if the interface is inconsistent

[KFAFSP]

We should write a design rationale, and add a What do we (not) want in this project section to the contribution guide.