Update of the Contribution Guide

[kaltokri]

Sipped from Discord:

Standard is ThisIsAVariable or a FunctionCall. _ThisIsPrivate. __ThisIsDelayed. ACLASSNAME. 
Local variables are faster than global ones. ipairs is faster for numerically(!) indexed tables

This contribution guide already exists. But we need to update it and move it to the Developer section.

[kaltokri]

We need to define conventions for file and path names in this guide, too!
This applies also for the demo missions repo.