add examples

[bigerl]

We need examples/elaboration for at least:

• [x] hashing a string and an int
• [x] using it in std::unordered_map and std::unordered_set
• [x] defining dice_hash for a custom type
• [x] is_unordered_container_v and is_ordered_container_v

[bigerl]

@LukasKerk what do you think: should we add example .cpp files in a subfolder examples or should we provide the description how to use it in the readme?

[LukasKerk]

17 does all of it. However it still needs some work.