Added contributing instructions.

CONTRIBUTING.md

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+## Reporting An Issue
+
+Please search for existing issues before opening a new one.
+
+### Where To Report
+
+Issues might be related either to syncthing, syncthing-android or both. The general rule of thumb is this:
+If the issue has to do with synchronization, discovery, the web interface, or can be reproduced on a desktop, it should be posted at [syncthing](https://github.com/calmh/syncthing/issues).
+
+If the issue is related to the Android UI, background service, or can't be reproduced on a desktop running the same syncthing version, it should be posted here.
+
+For general usage help or questions, you should post to [discourse](http://discourse.syncthing.net/category/support).
+
+### Bug Reports
+
+A bug report should include the following information:
+
+Description of the problem.
+
+Steps to reproduce:
+
+1. This is the first step
+2. This is the second step
+3. Further steps, etc.
+
+Observed behaviour and expected behaviour.
+
+syncthing-android version: x.x.x
+
+Device name and Android version:
+
+versions of involved syncthing nodes: vx.x.x, ...
+
+logcat: *link to file* (over the entire procedure, limited to the package `com.nutomic.syncthingandroid` if you can, [How to Logcat](http://forum.xda-developers.com/showthread.php?t=1726238) [Android Studio Logging Guide](http://stackoverflow.com/a/17648663/1837158)).
+
+config.xml: *link to file* (if it might be related, located in `/data/data/com.nutomic.syncthingandroid`, requires root)
+
+screenshots: *link to file* (only for UI problems)
+
+## Pull Requests
+
+Always welcome.
+
+Code should follow the [Android Code Style Guidelines](https://source.android.com/source/code-style.html#java-language-rules), with the exception that we use tabs, not spaces for indentation.
+
+Lint warnings should be fixed. If that's not possible, they should be ignored as specifically as possible.