docs: Add a note about the source code filename naming convention

Summary:
Do not use uppercase characters in source code filenames

This is a backport of Core PR13312

Test Plan:

make check
ninja all check-all bench-bitcoin

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3572