Added Cotribution Guidelines

Author: Michael Lohr

CONTRIBUTING.md

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+# Contribution Guidelines
+
+Let's help together and make a great list of free Unity scripts!
+
+If you know a resource which is not listed here, make a pull request or open an issue. But please read this file first.
+Please report broken links, outdated assets and spelling mistakes.
+
+## Notes:
+* The links are in no particular order.
+* Outdated assets are removed from time to time.
+
+## Requirements
+* All linked assets are free.
+* They don't have to be open source.
+* They have to contain scripts. Pure art assets are not allowed.
+* They are allowed to contain art assets.
+* They should work with the latest release of Unity.
+
+## Rules
+* You are allowed to suggest links to your own assets.
+* Look through all the other links and suggestions before subbmitting yours, to ensure yours is not a duplicate.
+
+## Link Suggestions
+* You can suggest a link to a free asset over [here](https://github.com/michidk/Unity-Script-Collection/issues/1).
+
+## Pull Requests
+* Make a new pull request for each suggestion.
+* Use the following format: \[name\]\(link\) - short description
+* Keep the descriptions simple as possible..
+
+Thank you for your help!