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

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

Top commit has no ACKs.

Tree-SHA512: 3f092f2e9d6d3baee75fd5e9fc396d67dbabe9dd0a78d8bda12b2bd3cc21d54139bd542761cae83d41c444c2be4ead063b0382bfa53c1c74e07743347f510a55