document how to test theme changes locally

**Enhancement**
The README.md should describe how to build this project and use it in another project. This would help contributors.

My development steps are
```
npm run build
npm run release:prepare
npm run dojo-package
```

Then within the project you want to test (e.g. widget-showcase)
`npm i ../../themes/dist/release`

FYI
Widgets showcase can take additional themes by adding them to the `main.ts` file

Update README.md with the listed build, release preparation, and package commands, then document installing the local release in another project such as widget-showcase. Check the widget-showcase main.ts reference to explain how additional themes are supplied, and verify that the documented steps match the local workflow.

Written by the indexing model from the issue text.