Add contribution guide

CONTRIBUTING.md

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+This is a quick guide on contributing to Isleward
+
+## Issues
+
+### Creating Issues
+* Use the integrated [Issue tracker](https://gitlab.com/Isleward/isleward/issues)
+* Before making a new issue, make sure the same issue is not already listed in the issue tracker
+* In case it is listed and you have some more info, for example on how to reproduce a bug, post a comment in the original issue
+* Use the `Bug` label for bugs
+* Use the `Suggestion` label for suggesting new features or content
+* Use the `Modding` label for modding related issues
+* Use the `Feature/Content` label for new features or content
+* Do **not** use the remaining labels unless you are working on the issue (see Workflow)
+
+### Workflow
+* First create an issue that relates to your changes if there's none yet (See Creating Issues)
+* Fork the Isleward project or update your fork
+* Make a new branch in your fork, preferably with the issue number and a very description in the name (e.g. `666-necromancer-class`, `123-crash-fix`)
+* Give the issue label a `In Development`
+* Make your changes
+* Before submitting a merge request, make sure to properly test your changes locally
+* Submit a merge request, in the merge request message, mention the task number, as mentioned in [Automatic issue closing](https://gitlab.com/help/user/project/issues/automatic_issue_closing.md) article.
+* Always merge into `isleward/staging` branch, **not** `isleward/master`
+* Remove the `In Development` label from the issue and give it an `In Testing` label
+* Wait for the merge request to be approved. If the merge request is denied, address the comments made in the request and submit again
+* After the merge request has been approved and merged, it'll get tested again in the `Staging` branch
+* When it's tested and works in `Staging`, assing a `Ready for Release` label to the issue
+* After the `Staging` branch is merged into `master`, the issue should get automatically closed. If not, please close the issue.