readme: add yarn install instructions to dev install instructions

[naterush]

When running the pip install .[...] install instruction, it fails with an error:

      > yarn run build:lib && yarn run build:nbextension && yarn run build:labextension

      Internal Error: @widgetti/jupyter-react@workspace:.: This package doesn't seem to be present in your lockfile; run "yarn install" to update the lockfile

This is fixable if you just yarn install before running the pip install command.

[naterush]

This fix got me to the next step, but still having trouble getting setup following the dev install instructions, specifically in the yarn run build:prod command that is called in some of the steps.

I'll spare you the details, but am I missing any critical install steps?

[maartenbreddels]

could you try with yarn<2?