Merge bitcoindevkit/bdk-cli#302: docs: Add documentation for adding new features

7e4a0cab549722321fedd40f1fb432c0f8d57020 docs: Add documentation for adding new features (Vihiga Tyonum)

Pull request description:

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  ### Description

  With the introduction of modular design architecture in the app (PR #278) , new contributors may find it challenging adding new features/commands. This PR adds a documentation section to provide them with an easy-to-follow guide.

  Fixes #298

  <!-- Describe the purpose of this PR, what's being adding and/or fixed -->

  ## Changelog notice

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  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk-cli/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  va-an:
    ACK 7e4a0cab549722321fedd40f1fb432c0f8d57020

Tree-SHA512: a865fdb0e57a93abe8d48dfd86e3c054e11026a29af76b377f766b47f828b8e1df18a2d73d2c41126436562ac710af4eaefa9b86a997eb90421a2254f8b6d6cf