README: clarify license

README

@@ -26,12 +26,35 @@ WARRANTY; without even the implied warranty of MERCHANTABILITY or
 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
 more details.
 
+Additional permissions under section 7 of the GNU General Public License
+(GPL) are hereby granted. The purpose of this exception is to allow
+non-GPL (including proprietary) logic cores to be used in conjunction
+with code generated by Migen in a design.
+
+"Generated" code refers to Verilog or VHDL code that has been obtained
+by converting FHDL structures using Migen.
+
+To "instantiate" is to include and connect another module as specified
+in section 12.1.2 of the IEEE standard 1364-2001 or in section 9.6 of
+the IEEE standard 1076-1993.
+
+(1) You have permission to propagate a work of generated code that
+    instantiates modules that are not generated code and do not comply
+    with the GPL.
+(2) You have permission to propagate a work of Verilog or VHDL code that
+    is not generated code, does not comply with the GPL and instantiates
+    generated code.
+
+The availability of this exception does not imply any general
+presumption that third-party software is unaffected by the copyleft
+requirements of the license of Migen.
+
 Unless otherwise noted, Migen's source code is copyright (C) 2011-2012
 Sebastien Bourdeauducq. Other authors retain ownership of their
 contributions. If a submission can reasonably be considered
 independently copyrightable, it's yours and I encourage you to claim it
 with appropriate copyright notices. This submission then falls under the
 "otherwise noted" category. All submissions must use a license
-compatible with the GPL.
+compatible with the GPL and the exception above.
 
   "Electricity! It's like magic!"