Add contribution guidelines

Author: emoacht

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 [Issues](https://github.com/emoacht/Monitorian/issues) to date. To search open and closed issues at once, remove `is:open` from default syntax in Filters box 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 give information on what is happening internally.
+
+- <ins>Be prepared to respond to a request after opening an issue.</ins> It is merely a startig point and you will be requested to provide additional information in most cases.
+
+## 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 suggestion.
+
+- This app's main focus and competence is brigtness of monitors. This app uses DDC/CI for this purpose, and not vice versa. The importance for keeping focused cannot be overemphasized.
+
+- Search [Issues](https://github.com/emoacht/Monitorian/issues) to date.
+
+- Explain the 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 implement the feature as much as possible. A feature which is not technically feasible cannot be added anyways.
+
+- <ins>Be prepared to respond to a request after opening a feature request or suggestion.</ins> It is merely a starting point and you will be requested to provide additional information in most cases.
+
+## 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 create 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#.