Clarify license for slides since they're not code

LICENSE

@@ -1,13 +1,12 @@
-Copyright 2015 Jérôme Petazzoni
-
-Licensed under the Apache License, Version 2.0 (the "License");
-you may not use this file except in compliance with the License.
-You may obtain a copy of the License at
+The code in this repository is licensed under the Apache License
+Version 2.0. You may obtain a copy of this license at:
 
     http://www.apache.org/licenses/LICENSE-2.0
 
-Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS,
-WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-See the License for the specific language governing permissions and
-limitations under the License.
+The instructions and slides in this repository (e.g. the files
+with extension .md and .yml in the "slides" subdirectory) are
+under the Creative Commons Attribution 4.0 International Public
+License. You may obtain a copy of this license at:
+
+    https://creativecommons.org/licenses/by/4.0/legalcode
+