Adds a .gitignore file

[louislelay]

Description

Adds a .gitignore file to avoid pushing MUJOCO_LOG.txt files as seen in #195. I also added __pycache__ as it can be generated after running pytest.

Checklist

• Added your name to CONTRIBUTORS.md (alphabetically by first name)
• Updated CHANGELOG.md:
  • Global changelog (if your change affects the overall repo)
  • [N/A] Model-specific changelog (if it affects a specific model only)
• [N/A] Followed the XML formatting/style guidelines (if editing MJCF)
• [N/A] Ran pytest test/ locally and ensured all tests pass
• Signed the Contributor License Agreement (CLA)