added contributing file

CONTRIBUTING.md

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+<!-- omit in toc -->
+# Contributing to Apollo
+
+First off, thanks for taking the time to contribute! ❤️
+
+## I Want To Contribute
+
+> ### Legal Notice <!-- omit in toc -->
+> When contributing to this project, you must agree that you have authored 100% of the content, that you have the necessary rights to the content and that the content you contribute may be provided under the project license.
+
+
+## Reporting Bugs
+Bug reports are always welcome. The more information you collect, the more helpful is your bug report. Please <b>do not submit</b> any feature requests.
+
+<!-- omit in toc -->
+### Before Submitting a Bug Report
+- Make sure that you are using the latest version.
+- Determine if your bug is really a bug and not an error on your side e.g. using incompatible environment components/versions
+- To see if other users have experienced (and potentially already solved) the same issue you are having, check if there is not already a bug report existing for your bug or error in the [bug tracker](https://github.com/nuclearfog/Apollo-Music/issues?q=label%3Abug).
+- Collect information about the bug:
+  - Android version
+  - Possibly your input and the output
+  - Can you reliably reproduce the issue? And can you also reproduce it with older versions?
+  - Logcat (optional)
+
+### Updating translations
+- If you want to add or update translation or fix a typo, feel free to open a pull request.
+
+### Submitting a Pull Request
+- For major changes only in consultation (open an issue first)
+