Instruct users to also install dotenv

[wwaaijer]

Hey there 👋

Since dotenv is a peer dependency now, to use dotenv-safe users will also need to install dotenv.
Currently this is not entirely clear from the installation instructions.

It's nice as a (first time) user to copy paste the installation instructions and have it work.
Currently, when blindly follow the instructions, they'll either get a warning from their package manager or a runtime error when trying to use dotenv-safe.

[rolodato]

Which package manager are you using? In my testing, only Yarn doesn't install peer dependencies by default.

[wwaaijer]

Hey, thanks for looking into this! When I encountered this issue I was indeed using Yarn.

Since the dotenv was not listed as an actual dependency for the other package managers, I though it might matter. But with some quick testing I was not able to come up with a breaking setup for the other package managers.

The PR is updated to only instruct Yarn users to also add dotenv.