[DOC] Add headers inclusion guidelines to the developer notes
[DOC] Add headers inclusion guidelines to the developer notes

Summary: Extracted from the recent changes to the header inclusions.

Test Plan:
  Read it.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2877

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