Update CONTRIBUTING instructions to match labview-doc-generator

Updated contributing instructions:

1. 'Development requirements' from LabVIEW 2014 SP1 32 bit to LabVIEW 2020 SP1. Note that I didn't specify bitness.
2. 'Modifying the code' - added instruction to make a topic on ZulipChat
3. Swapped Conventions for DQMH style guide as I believe they are all part of that. It now matches the LabVIEW Doc Generator repo CONTRIBUTING instructions

see #20 (closed)