Add versioning

[tabula-rosa]

Starting an issue so we can discuss a proposed way of doing versioning. How about using semantic versioning?
MAJOR.MINOR.VERSION

MAJOR >= 1 (since this repo is public now)

I was thinking we might want to anchor to IsaacGym's version somehow through ours (e.g., our MAJOR version is the same as IsaacGym's version.) But this might be a little complicated. So, we could just start with 1.0.0 and increment from there.

I like how albumentations does versioning -- it exists within the __init__.py file and is read by setup.py. So we could something similar for where we'd encode our version.
https://github.com/albumentations-team/albumentations/blob/5a3b2301f6ebfbece7a2e8c1e134fb08cfdfac86/albumentations/__init__.py#L3
https://github.com/albumentations-team/albumentations/blob/5a3b2301f6ebfbece7a2e8c1e134fb08cfdfac86/setup.py#L19

For development going forward, I'd suggest the following branches and development flow written here:

• master is the main development branch
• stable branches from master and contains tagged releases -> uploaded to PyPI. Only interacted with for doing releases and hotfixes. This is the production branch.
• topic- branches from master and is merged to master via PRs
• hotfix- branches from stable when an urgent bug affecting production needs to get fixed.