Merge bitcoindevkit/bdk#1871: docs: update CONTRIBUTING.md with section for new contr...

Merge bitcoindevkit/bdk#1871: docs: update CONTRIBUTING.md with section for new contributors

a3bc5dda295e45901cd5d11983c43ccac764e4d0 docs: update CONTRIBUTING.md with section for new contributors (Steve Myers)

Pull request description:

  ### Description

  This makes the discussion in #1762 easier to find for new contributors.

  ### 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)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

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