Update README

[Seismix]

Documentation improvements:

• The Contributing section in README.md has been rewritten to provide step-by-step instructions for prerequisites, installation, development, building, and browser configuration.
• Clarified the difference between basic and advanced settings in the extension, including a note that advanced CSS selector options are only available in the full settings page.
• Minor wording improvements and typo fixes in the README.md for better readability.

Development tooling:

• Replaced the previous dev:android script in package.json with a new Node.js script (scripts/dev-android.js) that builds the Firefox extension and deploys it to a connected Android device, making it possible to pass the ADB ID to the script.

[Seismix]

Closes #98