Merge bitcoindevkit/bdk-cli#237: feat: add completions subcommand

Merge bitcoindevkit/bdk-cli#237: feat: add completions subcommand

f32fc684d55fdb9b030c873f72fe10c4de1be99b docs(readme): add note about updating completions (va-an)
e173b476b59d286ae96a079292504dc4f3a42212 docs(completions): edit help output (Vadim Anufriev)
ba0bef15335808172f87b11e5f0707f9f1ace098 docs(readme): add completions block to readme (Vadim Anufriev)
c447159ef23bae5d0c54f7188d579d75379632be chore(clippy): fix clippy warnings again (Vadim Anufriev)
7c1303ad9544a578887c12809b18f1c7d0ecc068 chore(clippy): resolve clippy warngins (Vadim Anufriev)
82e662890f70170513ec3ce3ee825038e5f339f4 feat: add command for completions (Vadim Anufriev)

Pull request description:

  ### Description

  Resolves #234.

  Added `completions` subcommand to generate shell completions.

  ### How to test

  Install bdk-cli:
  ```bash
  cargo install --path . --all-features
  ```

  See how to install completions for your shell:
  ```bash
  bdk-cli completions --help
  ```

  For example, to install completions for zsh:
  ```bash
  bdk-cli completions zsh > ~/.zfunc/_bdk-cli
  ```

  After restarting the shell, completions should work with Tab.

  ### 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

  #### New Features:

  * [ ] I've added tests for the new feature
  * [x] I've added docs for the new feature
  * [x] I've updated `CHANGELOG.md`

ACKs for top commit:
  notmandatory:
    utACK f32fc684d55fdb9b030c873f72fe10c4de1be99b

Tree-SHA512: f033cb9624a9613356de8ef8f51576257f7f5ddd661d54853f3005294cd449d64810592317f3f62085adaccbc963f69792943bb2450b282cc13881b382855b1a