Add CLA and .cla-accepted file

.cla-accepted

+# Add you GitLab username and Git committer name in parentheses here
+@sebkur (Sebastian Kürten)

CLA.md

+# Contributor License Agreement
+
+If you are an employee and have created the contribution as part of your employment,
+you need to have your employer approve this agreement.
+If you do not own the copyright in the entire work of authorship,
+any other author of the contribution also needs to sign this.
+
+## Copyright License
+
+You hereby grant to Sublime Software Ltd,
+a worldwide, no-charge, royalty-free, non-exclusive, perpetual and irrevocable license,
+with the right to transfer an unlimited number of non-exclusive licenses
+or to grant sublicenses to third parties, under the copyright covering the contribution
+to use the contribution by all means, including, but not limited to:
+
+* publish the contribution,
+* modify the contribution,
+* prepare derivative works based upon or containing the contribution
+  and/or to combine the contribution with other materials,
+* reproduce the contribution in original or modified form,
+* distribute, to make the contribution available to the public, display
+  and publicly perform the contribution in original or modified form.
+
+## Free Software Pledge
+
+We agree to irrevocably (sub)license the contribution
+or any materials containing, based on or derived from your contribution
+under the terms of any licenses
+the Free Software Foundation classifies as [Free Software licenses](https://www.gnu.org/licenses/license-list.html)
+and which are approved by the Open Source Initiative as [Open Source licenses](http://opensource.org/licenses).
+See the [LICENSE](/LICENSE.md) file, for the current license used.