Add AUTHORS.md file

AUTHORS.md

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+A full developer contributors list can be found [here](https://github.com/vector-im/element-x-android/graphs/contributors).
+
+# Core team:
+
+The element.io Android developer team.
+
+# Other contributors
+
+First of all, we thank all contributors who use Element and report problems on this GitHub project or via the integrated rageshake function.
+
+We do not forget all translators, for their work of translating Element into many languages. They are also the authors of Element.
+
+Feel free to add your name below, when you contribute to the project!
+
+Name      | Matrix ID                   | GitHub
+----------|-----------------------------|--------------------------------------
+name      | @name:matrix.org            | [githubID](https://github.com/githubID)