README should call out feature differences vs. env_logger

[dimo414]

The README opens with:

for small to medium sized projects, that find env_logger to be somewhat lacking in features.

But skimming the rest of the document I don't see any sort of breakdown of which features simplelog provides that env_logger doesn't. It would be really helpful to call this out clearly on the landing page so potential users can quickly evaluate whether they would benefit from the features simplelog provides.