Mention the need to add the feature flag getrandom on README file

[viktaur]

Hi, I was following the instructions to get started with x25519-dalek and noticed that certain methods, such as EphemeralSecret::random, were unavailable. After some research, I found that the feature flag getrandom needs to be included to access them.

I think it could be helpful to mention this somewhere in the tutorial to avoid confusion among new users.

[tarcieri]

I assume you're talking about in the toplevel README.md here: https://github.com/dalek-cryptography/curve25519-dalek/tree/main/x25519-dalek#examples

Yeah, it should probably mention the getrandom feature (or maybe getrandom should be enabled by default)