Merge bitcoindevkit/bdk-cli#198: Add Justfile command runner

Merge bitcoindevkit/bdk-cli#198: Add Justfile command runner

98bef7ec55f421f7ae172fcfccd25725ad87486f chore(add-justfile): update README (Vihiga Tyonum)
ef84b928698d8ade83cc866e3ea78fffbd154307 chore: add Justfile for `just` command runner (Vihiga Tyonum)

Pull request description:

  ### Description

  This PR adds Justfile config file for the just command runner.

  ### Notes to the reviewers

  The default values for `datadir`, `rpcuser` and `rpcpassword` set in the Justfile are those used in the project.

  ### 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
  * [ ] I've updated `CHANGELOG.md`

ACKs for top commit:
  notmandatory:
    ACK 98bef7ec55f421f7ae172fcfccd25725ad87486f

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