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 81.91%. Comparing base (fc88144) to head (610fc5d).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #521      +/-   ##
==========================================
+ Coverage   81.84%   81.91%   +0.07%     
==========================================
  Files          25       25              
  Lines        6487     6535      +48     
  Branches      296      302       +6     
==========================================
+ Hits         5309     5353      +44     
+ Misses       1078     1075       -3     
- Partials      100      107       +7     

Flag	Coverage Δ
rust	81.91% <ø> (+0.07%)	⬆️

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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.

[notmandatory]

This doesn't need to be in 3.2 release so moved to 4.0, would just like to have it on master branch.

[oleonardolima]

ACK 3c4b3c5

I left a minor non-blocking nit.

[oleonardolima]

Suggested change

	See the .github/CODEOWNERS file for a current list of the project maintainers.
	See the [CODEOWNERS](.github/CODEOWNERS) file for a current list of the project maintainers.

[notmandatory]

thanks, fixed