Document differences across dotenv implementations

[allan2]

Ruby, JS, and Rust all have differences in their implementations.

This should be documented.

Relevant #11

[aidenfarley]

@allan2 would your comment on #11 be detailed enough to consider as a "preliminary" document for the differences across implementations?

If not, what are you looking for when it comes to this?

I would be more than happy to write this up, and any resources you may have pointing to the current behavior of other applications would be much appreciated.