Add doc on how to convert from gulp / grunt / react-scripts to just

@kenotron 2019-10-04 17:50:14发表于 microsoft/just Area: Documentation

gulp is a hop and a skip away from being a just project. Let's make it easy to convert!

grunt might be a bit more work...

react-scripts -> just-scripts should also be a 1 for 1 change if possible (keeping 80% of the functionality or maybe 100% with some contrib)