(Explicitly) putting EUDAQ under the LGPLv3 License

Dear colleagues,

thanks to your contributions, EUDAQ is a continuously evolving flexible and light-weight DAQ system, that is heavily used with the beam telescopes and could also serve other purposes.

Therefore, we think it is important to clarify the license under which EUDAQ is being distributed so that others interested in making EUDAQ part of their projects find a clearly termed basis for that.

As discussed previously, we have chosen the Lesser GNU Public License (LGPL) as this best expresses the collaborative spirit of scientific R&D in our opinion -- and it also follows the recent recommendations by CERN for new software projects.

However, since EUDAQ has been developed over such a long time with so many different contributors, we would like to hear your opinion or possible objections!

Also, if you prefer not to have your name/email shown in the newly added AUTHORS list included in this PR, please let us know!