Update secp256k1 README

Summary:
Now that secp256k1 is being bundled as it's own repo on Github,
it would be nice to have a more accurate and complete README.

Test Plan: Read it, make sure it's complete and accurate.

Reviewers: deadalnix, markblundeberg, jasonbcox, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4819