Initial draft of coding standards for OfficeDevPnP.Core

Some initial coding standards added to the ReadMe.md of the core API library.

These are based on the prevalent conventions existing within the code (which can be inconsistent in areas), and not necessarily my personal opinion. It would be good to have some debate whether these are acceptable or should be changed.

Once accepted, inconsistent areas of code can then be cleaned up (marking old methods as Obsolete where necessary).