README for requirements

The folder requirements contains different .yml files containing backend libraries/package which are used in DeepChem. Adding a README.md will be useful to easily understand what purpose each .yml file serves. Broadly,

• env_common.yml contains libraries related to scientific applications
• env_ubuntu.yml and env_mac.yml are dependencies which are supported only on those specific OSes.
• jax, tensorflow and torch folder contains installation procedure for deep learning backends