docs: add maintainers section to CONTRIBUTING.md

[notmandatory]

Description

I borrowed from the rust-bitcoin CONTRIBUTING.md file to add a maintainers section to our CONTRIBUTING.md file. The main points I wanted to clarify are:

1. a maintainer can merge a PR based on the reviews of other project contributors they trust to competently review and test changes. They do not need to do a detailed review and test each and every PR themselves.
2. a maintainer must ensure "rough consensus" is reached on the need for and correctness of a PR.
3. at least one maintainers must ACK a PR who is not the original author.

Notes to the reviewers

These changes match recent bitcoindevkit/bdk_wallet#521 and I'll propose the same change on our other repos.

Changelog notice

• Add maintainers section to CONTRIBUTING.md file.

Before submitting

• I followed the contribution guidelines
• This PR breaks the existing API

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.84%. Comparing base (e417c43) to head (fcf1e44).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2328   +/-   ##
=======================================
  Coverage   78.84%   78.84%           
=======================================
  Files          31       31           
  Lines        6060     6060           
  Branches      288      288           
=======================================
  Hits         4778     4778           
  Misses       1203     1203           
  Partials       79       79           

Flag	Coverage Δ
rust	78.84% <ø> (ø)

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