Add the mandatory license file to be consistent and proper

See the following mailing list thread for further details:
http://lists.qt-project.org/pipermail/development/2013-February/009810.html

Change-Id: Iec80c7cbeeafca7d2cdd63eb9c918d8dedbc6cd0
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

LGPL_EXCEPTION.txt

@@ -0,0 +1,22 @@
+Digia Qt LGPL Exception version 1.1
+
+As an additional permission to the GNU Lesser General Public License version
+2.1, the object code form of a "work that uses the Library" may incorporate
+material from a header file that is part of the Library.  You may distribute
+such object code under terms of your choice, provided that:
+    (i)   the header files of the Library have not been modified; and 
+    (ii)  the incorporated material is limited to numerical parameters, data
+          structure layouts, accessors, macros, inline functions and
+          templates; and
+    (iii) you comply with the terms of Section 6 of the GNU Lesser General
+          Public License version 2.1.
+
+Moreover, you may apply this exception to a modified version of the Library,
+provided that such modification does not involve copying material from the
+Library into the modified Library's header files unless such material is
+limited to (i) numerical parameters; (ii) data structure layouts;
+(iii) accessors; and (iv) small macros, templates and inline functions of
+five lines or less in length.
+
+Furthermore, you are not required to apply this additional permission to a
+modified version of the Library.