[CMMN] Provide some small examples

[margue]

Add some small examples to show crucial (and misleading) CMMN constructs

[tmetzke]

Hi Martin,

there are several examples for all supported CMMN operations present in the project now.
We encourage you to add further examples by raising a pull request with those if you would like to contribute.

Closing this as fixed.

Best,
Tobias