Add todo policy to documentation

[m273d15]

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Add the following policy to the documentation:

Always open a new issue if you added a TODO comment in your code

[m273d15]

I think it is more relevant to define a best-practice to write a TODO: A TODO should contain the information:

• What should be adjusted and (which is often missing) why. Without the "why", other develops are not able to evaluate whether the TODO is still relevant.