Update documentation to require NodeJS v15 or later for generateKey function

[arthur-clq]

Hi,

Thank you for creating this amazing library. I have a suggestion to indicate that Nodejs v15 or later is required to run generateKey because generateKeySync is only available on v15 or later.

I hope this would help others because I spent 10 minutes figuring out why its not working until I read through the Nodejs crypto docs.

[TABmk]

Hi, @arthur-clq

Thanks for letting me know about missing info
Just updated readme, docs and npm
v2.1.2 patch is on npm already