Break out copyright assignment

Pull copyright assignment to separate section of the PR template, and
explicitly state that contributions will not be accepted if they are not
contributed as Apache 2.0.

Author: unixorn

PULL_REQUEST_TEMPLATE.md

@@ -15,6 +15,10 @@
 - [ ] Add/remove a utility
 - [ ] Text cleanups/typo fixes
 
+# Copyright Assignment
+
+- [ ] This list is Apache 2.0 licensed, and I agree that the contribution in this PR is subject to that license. If you do not check this, I cannot add your contribution to the list.
+
 # Checklist:
 
 <!--- Go over all the following points, and put an `x` in all the boxes that apply. -->
@@ -25,4 +29,3 @@
 - [ ] I have read the **CONTRIBUTING** document.
 - [ ] All new and existing tests passed.
 - [ ] I have stripped leading **aws-** or **aws-lambda-** from the link name. This makes it easier to find things by name by preventing a big clusters in the A sections of the sublists.
-- [ ] This list is Apache 2.0 licensed, and I agree that my PR is subject to that license.