Avoid confusion about env registration

[Trenza1ore]

Hiii,
firstly thanks for making such an interesting project possible.

I've noticed that sometimes when first starting to use MinAtar, there is a big confusion:

Example code in README.md did not show that environments need to be registered into gymnasium before gym.make(...). Since there is no example code regarding gymnasium wrapper, README is the main source of info.

I have made the following minor changes to the code base:

• Add from .gym import register_envs in __init__.py for user friendliness
• Include import minatar and minatar.register_envs() to code blocks in README.md
• Also removed an incorrect ```` in README.md that breaks some Markdown renderers

It would be nice if you can accept this PR to reduce confusion, since ALE/Atari register environments at import, it is not uncommon for newcomers to assume the same here without further clarification in README.

What would also be nice to have is some example code regarding gymnasium wrapper, perhaps an SB3 example? I'm happy to help if that's also desirable.