Add license file to the repository

[bkmgit]

No description provided.

[PaulBoersma]

Both GPL 2 and GPL 3 are in the main folder. Should they be somewhere else?

[bkmgit]

Typically license files are in the topmost folder, or information on where the license files are is placed in the topmost folder. A similar issue is raised in #2439 The README.md has the following:

2.1. License

All of the code is available on GitHub under the [GNU General Public License](http://www.fon.hum.uva.nl/praat/GNU_General_Public_License.txt). Of course, any improvements are welcomed by the authors.

Perhaps this could be changed to

2.1. License

All of the code is available on GitHub. Source code is available under the GNU General Public License v3 and the GNU General Public License v2. The text of these licenses is in the main folder. Of course, any improvements are welcomed by the authors.

If the above is suitable, will close https://github.com/bkmgit/praat/tree/add-license Explaining which portions are under GPLv3 and which under GPLv2 would also be helpful. In addition, explaining that bundled software has other licenses would also be good.