Update README

Accept contributions through GitHub pull requests.

CONTRIBUTING.md

@@ -2,12 +2,9 @@
 
 ## Submitting Patches
 
-At this point, we don’t accept pull requests for the Threema apps via GitHub.
-However, if you would like to provide a code contribution, you can send patches
-via e-mail.
-
-Please refer to <https://threema.ch/open-source/contributions> for more
-information on how to contribute.
+We accept GitHub pull requests. Please refer to
+<https://threema.ch/open-source/contributions>
+for more information on how to contribute.
 
 ## Bug Reports / Feature Requests
 

README.md

@@ -185,18 +185,16 @@ The code of the actual Android app is mainly located in the
 
 ## <a name="contributions"></a>Contributions
 
-For the time being, we do not accept contributions via GitHub. If you want to
-make a contribution, first make sure that you have signed the
-[CLA](https://threema.ch/open-source/cla/).
-You can then submit patches over email by following the instructions at
-[threema.ch/open-source/contributions/](https://threema.ch/open-source/contributions/).
+We accept GitHub pull requests. Please refer to
+<https://threema.ch/open-source/contributions>
+for more information on how to contribute.
 
 
 ## <a name="license"></a>License
 
 Threema for Android is licensed under the GNU Affero General Public License v3.
 
-    Copyright (c) 2013-2020 Threema GmbH
+    Copyright (c) 2013-2021 Threema GmbH
 
     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU Affero General Public License, version 3,