Update instructions for how to work on codebase

Aims to fix problems associated with #390

> You can get the library to re-compile on change by running yarn start from the root rimble-ui directory. This will watch the /src directory for changes and recompile the library that is used in the demo app. So you will need 2 terminals, both running yarn start - one for the library and one for the react app.

> We need to update our instructions for how to work on the codebase. I think that should be the outcome to close this issue.