Add GPLv3+ license

LICENSE

@@ -1 +1,5 @@
+You may use these files under either of these licenses:
+
+GPLv3+
+
 CC by SA 4.0 International

README.md

@@ -1,5 +1,5 @@
 This project contains the parts to 3D print a case/shell for the Olimex
-TERES-I Do-It-Yourself Free Open Source Hardware and Software laptop:
+TERES-I Do-It-Yourself Free Software and Open Source Hardware laptop:
 
 https://www.olimex.com/Products/DIY-Laptop/
 
@@ -16,9 +16,9 @@ Set bed size to 295mm
 
 
 This project is independent of Olimex.
-All files, unless otherwise noted, are released under the
-CC by SA 4.0 International
-License.
+
+All files, unless otherwise noted, are released under the GPLv3+ and the
+CC by SA 4.0 International License.
 
 
 Copyright (C) 2018 Fork Sand, Inc.