Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Filing an issue
+
+### Must read
+* If you aren't sure, you can ask on the [**forum**](http://forum.qbittorrent.org) or read our [**wiki**](http://wiki.qbittorrent.org) first.
+* Do a quick **search**. Others might already reported the issue.
+* Write in **English**!
+* Provide **version** information: (You can find version numbers at menu `Help -> About -> Libraries`)
+  ```
+qBittorrent:
+Qt:
+libtorrent:
+boost:
+OS version:
+
+  ```
+* Provide **steps** to reproduce the problem, it will be easier to pinpoint the fault.
+* **Screenshots**! A screenshot is worth a thousand words. just upload it. [(How?)](https://help.github.com/articles/file-attachments-on-issues-and-pull-requests)
+
+### Good to know
+* **Patience**. The dev team is small and resource limited. Devs finding their free time, analyzing the problem and fixing the issue, it all takes time. :clock3:
+* If you can code, why not become a **contributor** by fixing the issue and open a pull request? :wink:
+* Harsh words or threats won't help your situation. What's worse, your complain will (very likely) to be **ignored**. :fearful:
+
+
+# Opening a pull request
+
+### Must read
+* Read our [**coding guidelines**](https://github.com/qbittorrent/qBittorrent/blob/master/CODING_GUIDELINES.md). There are some scripts to help you: [uncrustify script](https://gist.github.com/sledgehammer999/1cb1d9224d7fec8fa632), [astyle script](https://gist.github.com/Chocobo1/539cee860d1eef0acfa6), [(related thread)](https://github.com/qbittorrent/qBittorrent/issues/2192).
+* Keep the title **short** and provide a **clear** description about what your pull request does.
+* Provide **screenshots** for UI related changes.
+* Keep your git commit history **clean** and **precise**. Commits like `xxx fixup` should not appear.
+* If your commit fix a reported issue (for example #4134), add the following message to the commit `Closes #4134.`. Example [here](https://github.com/qbittorrent/qBittorrent/commit/a74bac20c4e8de9776bf9bb77fdc7526135d1988).
+
+### Good to know
+* **Search** pull request history! Others might already implemented your idea and is waiting to be merged (or got rejected already). Save your precious time by doing a search first.
+* When resolving merge conflicts, do `git rebase <target_branch_name>`, don't do `git pull`. Then you can start fixing the conflicts. Here is a good explanation: [link](https://www.atlassian.com/git/tutorials/merging-vs-rebasing).