[DOC] Update the subtree section from the developer notes

Summary:
This section is not applicable to Bitcoin ABC since there is no subtree.

The documentation is updated to give advices for making changes to third
party libraries instead, and secp256k1 is removed from the list.

Test Plan: Read it.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5396