Add guidelines for graph conventions around read-only properties

[corranrogue9]

I am adding a new file to document some of the graph conventions we have around properties. This PR is documenting the convention that read-only properties can be "updated" so long as they are being set to the same value that they currently have.

Future changes to this file will contain conventions around default property sets, and conventions for the requiring $select, which I will go to the council meeting with proposals for. The current change is documenting something that is already established and has come up a few times in the last several months.