added contribution guidelines

Contribution Guidlines.md

+# Contributing
+
+Your contributions are always welcome!
+
+## Guideline
+
+* Add one link per Pull Request.
+    * Make sure the PR title is in the format of `Add project-name`.
+    * Write down the reason why the contribution is suitable.
+* Add the link: `* [project-name](http://example.com/) - A short description ends with a period.`
+    * Keep descriptions concise and **short**.
+* Add a section if needed.
+    * Add the section description.
+    * Add the section title to Table of Contents.
+* Search previous Pull Requests or Issues before making a new one, as yours may be a duplicate.
+* Check your spelling and grammar.
+* Remove any trailing whitespace.