Add requirements to file

[fabianmoss]

Currently, the README states which requirements are necessary to install the library. This leaves to much work for the user. A better way is, to include a file requirements.txt that contains all needed libraries that we need, e.g.

pandas==0.24.2
matplotlib==3.0.3

Then, the installation instructions have to be changed to

pip install pitchplots -r requirements.txt