Clarify that non sfml-graphics libraries need to be added to CMakeList

[Skrivmaskinen]

The CMakeList includes the sfml-graphics library, but not the others, such as sfml-audio. A new user would not understand that these are different libraries that need to be specified independently.

Suggestion 1: Add the rest of the libraries to the CMakeList.txt
Suggestion 2: Add instructions to the README.md for how to change CMakeList.txt to link to these libraries