Merge bitcoindevkit/bdk#1584: docs: update CONTRIBUTING.md

6b881f8ab46bd0f3c612e522df21dc48ae5e6955 docs: update CONTRIBUTING.md (valued mammal)

Pull request description:

  Update `CONTRIBUTING.md`

  * Made small corrections and added some things that were missing. If there's something else you think should be included let me know.

  closes #1394

  ### Changelog notice

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

ACKs for top commit:
  LagginTimes:
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  notmandatory:
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  storopoli:
    ACK 6b881f8ab46bd0f3c612e522df21dc48ae5e6955
  oleonardolima:
    ACK 6b881f8ab46bd0f3c612e522df21dc48ae5e6955

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