Be more clear about the license of the code

This adds the LICENSE.txt file with the text of the LGPL v2.1 license and adds back the original note to the RebelAllianceMod sketch explaining that it is under that license. Because of the nature of the LGPL that was used in the original TenTec code, our community modifications must also be licensed under the LGPL.  The openQRP keyer code that we incorporated is also LGPL, so our project needs to have a compatible license to use that code.

This doesn't change the license of the RebelAllianceMod code, it just makes it more clear what license terms it always had. Having the license terms in every file with the full text included is common in open source projects.