Update license to accomodate copy-pasting source code policies

[2DSharp]

The license states the usage of the complete software packaging. A fourth point on the terms of copy-pasting source code from this project could be added. Adding the notice (along with the author credits) could be mandated if someone has to copy-paste any code snippet.

[hellozee]

How can we make sure, whether the code snippet is copied from the current source code or not?

[2DSharp]

We don't need to make sure. This is not our responsibility. This is the responsibility of the person copying it to put the license information on top of their destination source pages.