Add setup instructions to README

[Brian-Magnuson]

The initial readme has few instructions on how to set up a development environment for modifying the extension. This adds a few basic setup instructions, which can benefit people new to Chrome extension development. This also adds some brief information on how the extension works once it is loaded and some important files in the project when viewing the codebase for the first time.

• Adds a features section for brief information on how the extension works.
• Adds a setup section with step-by-step instructions on how to install dependencies and setup Chrome to load the extension for development.
• Adds a testing sub-section for information on how to run Jest tests.

[kepi]

Thanks @Brian-Magnuson this was definitely needed.

Btw. tests are run with Vitest since last version, but README doesn't specific specific technology, so no problem with that.