Create contributor_guidelines

[StatisMike]

As most of the syntax in the package is based on internal functions, there is a need to create clear contributor guidelines for a chance of a new contributor joining the project. It should describe the general workflow when adding support for another GoogleSheet class:

dgs4Obj objects

• creation of constructor function
• optional addition of the generator function
• optional addition of the dgs4_listinize() method
• creation of unit tests

dgs4Req objects

• creation of constructor function
• creation of unit tests