Mention the license notice for new files in CONTRIBUTING

Change-Id: I0785afb167f71ee545053c17b05e25e40efc05e1
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/77287
Reviewed-by: Adam Langley <agl@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 606475e..e6f9471 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -65,6 +65,23 @@
 all past contributors. If you previously made a contribution, you are likewise
 welcome to send us a patch to be added, or request that we add you.
 
+After the copyright lines, files should include the license notice described in
+the Apache 2.0 appendix. Thus new files should begin with the following header:
+
+    // Copyright YEAR The BoringSSL Authors
+    //
+    // 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
+    //
+    //     https://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 small print
 Contributions made by corporations are covered by a different agreement than
 the one above, the