Choose and add open source license.

[kfogel]

Thanks for the great tool! If you tell me what open source license you'd like it distributed under, I'd be happy to supply a pull request that creates the appropriate LICENSE.md file and source file copyright notice headers. (Or if you're not sure which license you'd like, let me know and we can discuss options. https://choosealicense.com/ may be useful for this.)

[kfogel]

Hi, @gbilder. Based on https://github.com/CrossRef/event-data-reverse/blob/master/LICENSE, I'm guessing MIT License? If you'd like a PR to add that, just let me know.

[jirihradil]

The software uses iText 5.0.3 which has its own licence. Do we need an extra licence for iText to use pdfstamp?