Add explicit license statement to PR template

See https://opensource.stackexchange.com/questions/4267/how-does-contributing-to-a-floss-project-work-from-a-legal-perspective/4268

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,12 @@
 <!-- This is a template for your Pull Request. This are just some suggestions for you. You do not have to use all of them. -->
 
+<!-- **Attention:** Please tick this box to agree to release your code under the projects license. Otherwise your PR cannot be accepted. -->
+* [ ] I agree to release the changes in this PR under the Zlib/libpng license as shown in the  [LICENSE file](https://github.com/PrivateBin/PrivateBin/blob/master/LICENSE.md#zliblibpng-license-for-privatebin).
+
+<!-- Alternatively uncomment and tick this box to release your code under public domain:
+* [ ] To the extent possible under law, I have waived all copyright and related or neighboring rights to this PR and publish it as public domain.
+-->
+
 <!-- If your PR fixes an issue, mention it here. You can also just copy the URL - GitHub will convert it for you. -->
 This PR fixes