Add how to develop section in the Readme

[kaiomagalhaes]

1. export VERSION={your own descriptive release name} i.e. export VERSION=edlocaldev
2. make release (creates a release for you through GitHub actions. Give it a few minutes, can check the progress on GitHub.)
3. convox rack update {release name} -r {local rack name} i.e convox rack update edlocaldev -r dev
4. export IMAGE=convox/convox:{release name} && export RACK={rack name} i.e. export IMAGE=convox/convox:edlocaldev && export RACK=dev
5. Make local code changes
6. make dev (builds your updated code, pushes the image, restarts your local Rack components
7. Check and test your Rack for your updates. Go to 5 and repeat as necessary.