Custom environment

Adding custom environment by following the README.md in examples/extending/task instructions doesn't work. Doing it by this method results in circular import of Task class. As this is an essential and vital part of BenchMARL, I kindly seek help in the process and suggest improvements to README.md for clarifying the intended use and provide greater assistance.

Start with README.md and the instructions under examples/extending/task, then reproduce the custom-environment setup to locate the circular import involving Task. Done means the documented process works without that import failure and README.md clearly explains the intended use and setup.

Written by the indexing model from the issue text.