Add contribution guidelines

CONTRIBUTING.md

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+# Contributing to Monitorian
+
+Thanks to your interest in contributing to this project.
+
+## 1. Issues
+
+Please take following steps to open an issue.
+
+- Read through [Detection of external monitors](https://github.com/emoacht/Monitorian#detection-of-external-monitors) and [Reporting](https://github.com/emoacht/Monitorian#reporting) in readme.
+
+- Search open and closed issues to date. To search both issues at once, remove `is:open` from default syntax and use only `is:issue` syntax.
+
+- Include all information necessary for a reader who has no knowledge about your monitor or other devices to understand the issue. Include how to reproduce the issue as well.
+
+- Add logs. A video clip is not so helpful because it will not have information on what is happening internally.
+
+- Opening an issue alone will not mean you completed reporting. <ins>Be prepared to respond to a request after opening.</ins> In most cases, you will be requested to provide additional information.
+
+An issue which does not include any meaningful clue will be closed on sight. It is because such person would hardly provide enough information even if asked to do so.
+
+## 2. Feature requests or suggestions
+
+Do not open feature requests or suggestions in __Ideas__ of __Discussions__ in this project. It is for aggregating them in __Issues__.
+
+Please take following points into account to open a feature request or suggesion.
+
+- This app's main focus and competence is brigtness of monitors. This app uses DDC/CI for this purpose, and not vice versa. This point is very important.
+
+- Search open and closed issues to date.
+
+- Explain necessity and added value of the feature. Describe how the feature could be useful in actual use cases or scenarios. An existing feature of other app will not be reason for adding the same to this app.
+
+- Include technical information to accomplish the feature. A feature which is not technically feasible cannot be added anyways.
+
+- <ins>Be prepared to respond to a clarification after opening.</ins>
+
+## 3. Pull requests
+
+### 3.1 PR for language
+
+- Read [Globalization](https://github.com/emoacht/Monitorian#globalization) in readme.
+- Use a phrase used by the OS as much as possible.
+- Do not try to explain everything. You cannot present a flawless expression in a short sentence like menu item.
+
+### 3.2 PR for functionality
+
+- Open an issue and get agreement for your proposed change before starting your work.
+- It is recommended to use the latest features of C#.