docs: add contributing guidelines

contributing.md

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+# Contribution Guidelines
+
+Please note that this project is released with a [Contributor Code of Conduct](code-of-conduct.md). By participating in this project you agree to abide by its terms.
+
+Please ensure your pull request adheres to the following guidelines:
+
+- Alphabetize your entry.
+- Search previous suggestions before making a new one, as yours may be a duplicate.
+- Suggested README tools should be beautiful or stand out in some way.
+- Make an individual pull request for each suggestion.
+- New categories or improvements to the existing categorization are welcome.
+- Keep descriptions short and simple but descriptive.
+- If you are the tool owner and want to update the description of your tool entry which I have added, then you are welcome ✌.
+- Start the description with a capital and end with a full stop/period.
+- Check your spelling and grammar.
+- Make sure to use `.editorconfig` for formating.
+
+Thank you for your suggestions!