Update Contributing instructions

Since the Toolkit is now in LabVIEW 2020, can update Development Requirements from LabVIEW 2014 SP1 32 bit to LabVIEW 2020 SP1.

See discussion in #Antidoc > LabVIEW Development version for contributions @ 💬

Can also update 'Modifying the code' to mention to first discuss the issue in Zulipchat's #Antidoc Add-on - LabVIEW Toolkit, to match the other contribution pages. E.g. if I copy the one from the LabVIEW Doc Generator repo 1. If you want to add feature or change behavior, first start a new topic in our Zulip chat room and then create an issue so we can discuss it.

I also see that the Conventions section could be replaced with for consistency.