Update CONTRIBUTING.md
Update CONTRIBUTING.md

Added note about using the `develop` branch, plus a few other minor editing tweaks
matt9mg/concrete5,avdevs/concrete5,TimDix/concrete5,MichaelMaar/concrete5,matt9mg/concrete5,avdevs/concrete5,MichaelMaar/concrete5,TimDix/concrete5,TimDix/concrete5,MichaelMaar/concrete5,avdevs/concrete5,matt9mg/concrete5