Add the API 100 reference to the README

[WSQS]

It would be helpful to document the correct API 100 reference for module development in the README. In particular, please include the specific API commit, tag, or version that developers should use, so they can target the correct implementation without guesswork.

[JingMatrix]

Nice proposal.

Currently, in this page: https://github.com/JingMatrix/Vector/tree/master/xposed, GitHub UI shows you current commit of libxposed API (as a git submodule) we are using.

[WSQS]

Okay, it's truly a detail that is easy to ignore. But thanks for this information. I really find the line that define the version and blame it to find this commit for Vector 2.0. lol

[JingMatrix]

I have updated the doc via #639