put warning message on top of README.md

[louisdecharson]

Hi @dysfunc, Thank you for this repo, the emojis are really nice. I think it would be worth adding the important note that despite the repo's name, these are not ASCII emojis on top of the README (so if you are really looking for an ASCII emoji, you don't have to scroll until the last one to learn that this is not going to fulfil your need). 😉