docs: Add documentation for adding new features

[tvpeter]

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

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.89%. Comparing base (047bbbe) to head (7391aad).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #302   +/-   ##
=======================================
  Coverage   56.89%   56.89%           
=======================================
  Files          22       22           
  Lines        3661     3661           
=======================================
  Hits         2083     2083           
  Misses       1578     1578           

Flag	Coverage Δ
rust	56.89% <ø> (ø)

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