Potential improvements to DEVELOP.md

[silviubogan]

The steps in DEVELOP.md miss one thing. At the end, before eventually running yarn test, for yarn test to work, one must put
this line over the scripts.test line in package.json:

"test": "razzle test --env=jsdom --passWithNoTests --watchAll",

This line previously was:

"test": "razzle test --env=jsdom --passWithNoTests",

The new thing is --watchAll, without which this error shows up when running yarn test:

Determining test suites to run...
--watch is not supported without git/hg, please use --watchAll 
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.