Add dotenv import to README

[razzius]

When reading the README, I appreciated the simple and to-the-point documentation of how to get a dotenv value:

Dotenv dotenv = Dotenv.load();
dotenv.get("MY_ENV_VAR1")

However the full path of the import isn't anywhere in the readme (I found it in the example code). I figured other people might be looking for that in the readme, so I just added that line to the above example:

import io.github.cdimascio.dotenv.Dotenv;

I know some IDEs can import symbols like Dotenv automatically, and I'm also aware that those lines won't run directly since the import goes outside the class and the Dotenv.load() would go inside some sort of main method, so I'm open to adding a ... or otherwise restructuring that code snippet. What do you all think?