[TODO]: Better documentation for common Q&As and also for developers

[oscarwumit]

Notes
From time to time we get similar issues from different users, so it seems to be helpful to create a Q&A section in the documentation page or somewhere else. Examples of commonly asked questions are (not comprehensive and we should keep a record):

• Installation chemprop properly with GPU (sometimes users installed the CPU version without realizing it, and subsequently complain about the training time to be really long)
• Several users have asked about general best practices and guidance to pick hyper-parameters and settings for different tasks (regression, classification) and on how to best plan out hyper-parameter optimization in general.
• How to control the randomness of chemprop, e.g., difference between seed, pytorch_seed, and other randomness in the program.

Also, sometimes we implement certain features meant for developer use only, and we should document the details. For example: we might want to recommend the developer to do developmental install (pip install -e option) and install pytest when setting up chemprop environment. Also, certain test might only work on CPU and might fail in GPU environment, and we should note that for the developers.

[kevingreenman]

Maybe we should keep developer docs in our existing private repo unless we're thinking of having many people from outside our group becoming active developers? Or we could also try to separate things that anyone making a PR might need to know from things that only internal people should know.

[oscarwumit]

That could work for the developer docs.