Adding AUTHORS and changing licensing terms from 'GPLv2 or later' to 'GPLv3 or later' to comply with components available only under 'GPLv3 or later'. Details:

Initial releases by Jonathan Westhues in 2005-2007 were made under GPL
"either version 2 of the License, or (at your option) any later version"

Since then, a number of dependencies and files were integrated to the project, but under a "GPLv3 or later" license. These components are reveng, desfire_crypto and lrpcrypto. Note that compnents cryptorf, hitag2crack and fpga-xc3s100e are also under GPLv3+ but not bound to the firmware or client source code.

Therefore, we must mechanically upgrade the license of the whole project to GPLv3+, as made possible by the initial licensing terms and as mandated by the inclusion of GPLv3+ code.

Still to do: update each source file header:

    Copyright (C) Proxmark3 contributors. See AUTHORS.md for details.

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.

AUTHORS.md

@@ -0,0 +1,12 @@
+Initial author of the Proxmark3 code is Jonathan Westhues, starting in August 2005.  
+His latest release was done in May 2007 and is available [here](https://cq.cx/dl/proxmark3-may23-2007.zip) (copy available [here](http://proxmark.org/files/J.Westhues/)).
+
+Initial copyright notice is therefore:  
+Copyright (C) 2005-2005 Jonathan Westhues
+
+Since then, each contribution is under the copyright of its respective author.
+
+A few releases were done by the Proxmark community between 2007 and March 2009 before using version control.  
+The last one which served as basis for version control, under SVN then migrated to Git, was the `20090306_ela` release by Edouard Lafargue. See the first commit of this repository.
+
+Since then, each contribution is tracked by the Git history. See the output of `git shortlog -nse` for a full list. See also [the Contributors page on Github](https://github.com/RfidResearchGroup/proxmark3/graphs/contributors).

README.md

@@ -216,3 +216,23 @@ If you need to refer to a specific state of the repository, use a commit number
   note = {Accessed: commit 12327f71a27da23831901847886aaf20e8ad3ca0}
   note = {Accessed: 2021-01-01}
 ```
+
+## Copyright and licensing terms
+
+Each contribution is under the copyright of its author. See [AUTHORS](AUTHORS.md).
+
+The Proxmark3 source code is covered by the following licensing terms, usually referred as **GPLv3 or later**.
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+A copy of the GPLv3 is available in [LICENSE](LICENSE.TXT).  
+
+Some dependencies may be under other free licensing terms compatible with the Proxmark3 licensing terms, see their respective description.