Add licensing exception that allows linking with Minecraft

Without this exception, people will not be able to distribute this
mod's JAR artifacts without any potential legal issues because they
contain proprietary components from Minecraft. I definitely will not
sue any distributor of the artifacts, but adding this exception would
help anyone who wants to distribute the artifacts be more confident.

LICENSE-MCE.txt

@@ -0,0 +1,25 @@
+HBW HELPER MC EXCEPTION
+
+This HBW Helper MC Exception ("Exception") is an additional permission under
+section 7 of GNU General Public License version 3 ("GPLv3").  It applies to
+this entire Program.
+
+As a Minecraft mod, some parts of HBW Helper have to be linked with Minecraft
+itself directly, rather than Minecraft Forge.  Minecraft is a
+GPLv3-incompatible proprietary program.  Linking this mod with it will produce
+a covered work with non-free components from Minecraft, and conveying the
+resulting work may cause legal issues surrounding GPLv3.  The purpose of this
+Exception is to allow conveying of such resulting work.
+
+When you convey a copy of a covered work, GPLv3 grants you the right to remove
+this Exception.  To promote subsequent contributions and enhancements to the
+covered work, you are encouraged to preserve this Exception in the conveying so
+others can keep improving the work and distributing any work based on it,
+though you are not obligated to do so.
+
+1. Grant of Additional Permission.
+
+If you modify this Program, or any covered work, by linking or combining it
+with Minecraft (or a modified version of that library), containing parts
+covered by the terms of Minecraft End User License Agreement, the licensors of
+this Program grant you additional permission to convey the resulting work.

README.md

@@ -59,15 +59,19 @@ instructions and contribution guidelines, in
 
 ## License
 
-HBW Helper 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
+HBW Helper is free software: you can redistribute it and/or modify it under the
+terms of the GNU General Public License (GPL) as published by the Free Software
 Foundation, either version 3 of the License, or (at your option) any later
 version.
 
 HBW Helper 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 [`LICENSE.txt`](LICENSE.txt) for more details.
 
+Under Section 7 of GPL version 3, you are granted additional permissions
+described in the HBW Helper MC Exception. See
+[`LICENSE-MCE.txt`](LICENSE-MCE.txt) for more details.
+
 ## Obtaining Source Code of This Mod
 
 You can download source code of this mod as how you would when you download