Removes python cache and updates README to tell about `source ./challenge/bin/activate`

Just did a simple `find . -name "*.pyc" -delete` so that we could stay away from this `.pyc` files.

I don't know if flask is also designed for python 3.x. If so we could also add `__pycache__/` folder to the .gitignore.

What do you think @eder?