Add requirement.txt

[rpanai]

Hi, I'm trying to build a conda-forge package out of this library but I've some troubles. Do you mind to ad a requirements.txt as this?
If neede I can eventually make a PR.

[tailaiw]

@rpanai We didn't list a requirements.txt because we list all the dependencies in setup.cfg and that is enough for pip installation. I never made conda packages before but great to hear you are adding ADTK to conda-forge. Feel free to create a requirements.txt based on the dependency list in setup.cfg and open a PR.