Update docs for nimbus-ui readme

[eliserichards]

Update the yarn development instructions here. These seem to be out of date

┆Issue is synchronized with this Jira Task

[eliserichards]

make check runs this command to run the .test.ts and .test.tsx tests:

yarn workspace @experimenter/nimbus-ui test:cov

looks like it needs to be run from the /app folder

[yashikakhurana]

not valid, closing this