Added support for Docker

- Updated SetUp.md with a "Run locally with Docker" section
- Moved requirements.txt to the root of the repo since some hosting
  services require the file to be there (eg. Heroku https://elements.heroku.com/buildpacks/heroku/heroku-buildpack-python)

Dockerfile

@@ -0,0 +1,12 @@
+# syntax=docker/dockerfile:1
+
+FROM python:latest
+
+WORKDIR /api
+
+COPY requirements.txt requirements.txt
+RUN pip3 install -r requirements.txt
+
+COPY api/ .
+
+CMD [ "python3", "spotify.py"]