Add notice in the contributions file

CONTRIBUTIONS.md

 
 ## List of licenses accepted
 
+Preferred license is Apache-2:
+
+~~~C
+/*-
+ * Copyright [year] [your name]
+ *
+ * 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
+ *
+ *   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 2 clause version of the BSD license