Overhaul README.md

Summary:

• Update feature list
• Be more positive about the state and quality of the library
• Mention ECDSA key operations explicitly in short library description
• Say "secret key" instead of "private key
• Define "experimental"

Co-Authored-By: Gregory Maxwell <greg@xiph.org>

This is a backport of libsecp256k1 PR703

Depends on D5377

Test Plan: Read.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5383