[SECP256k1] Update the README to include the CMake build instructions

Summary:
Also updated the autotools code block to remove the confusing leading
$, and discourage in-tree build.

Note: there is no install target for CMake at the moment.

Test Plan: Read the doc and test the instructions.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4857