Add note about AlertifyJS having GPL license to readme, or change license

[agvald]

AlertifyJS

If you are using AlertifyJS in a closed source project you should buy a licence. https://alertifyjs.com/tutorials.html

I think, to be on the safe side, this gem should have been GPL licensed, since you inlucde a GPL licensed library the entire gem should be GPL licensed not to violate the GPL of the AlerifyJS. And that would make it easy for everyone using this gem to include AlertifyJS notice that this is GPL licensed.

Anyhow, if you don't agree with that, could you at least add a warning into beginning of the readme? Telling that any program that integrates with AlertifyJS has to be released open source, under same conditions as the AlerifyJS gem. Or that you have to buy an AlertifyJS licence.