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 two maintainers must ACK a PR who is not the original author.

Notes to the reviewers

If these changes work for bdk_wallet I'll propose the same change on our other repos. I will also reach out to current maintainers to suggest additional people who they would like to add (and are willing to be) maintainers. Having only two per repo does seem to be enough, the rust-bitcoin team has eight people with maintainer rights.

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 81.84%. Comparing base (fc88144) to head (3c4b3c5).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #521   +/-   ##
=======================================
  Coverage   81.84%   81.84%           
=======================================
  Files          25       25           
  Lines        6487     6487           
  Branches      296      296           
=======================================
  Hits         5309     5309           
  Misses       1078     1078           
  Partials      100      100           

Flag	Coverage Δ
rust	81.84% <ø> (ø)

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[notmandatory]

I added this to the next milestone but if it doesn't get adequate review can move to the next one.