CLA draft for discussion

I did use the "project harmony" mentioned in Issue #157, to create some basic TW5 CLA's using there templates.

You can see the result in my activate-CLAs branch: https://github.com/pmario/TiddlyWiki5/tree/activate-clas

There is a new directory `licenses` and I did modify `contributing.md` to describe, how signing should work. The links in contributing.md actually point to the master branch. so you'll need to manually navigate to this dir. 

@Jermolene, @natecain, @Brennall, @tobibeer, @KenanY, 
what do you think. Would you sign this CLA?

[This link](https://github.com/pmario/TiddlyWiki5/commit/dddb8bbccaf5f5b1f1793c7bb6266e19268dcbfd) also shows the diff between "individual" and "entity" CLA.