Add contribution guidelines.

CONTRIBUTING.md

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+# Contribution Guidelines
+
+Please ensure your pull request adheres to the following guidelines:
+
+1. Please search previous issues before making a suggestion, as it may be a duplicate.
+
+2. Libraries that are Composer installable, PSR compliant, and tested are preferred.
+
+3. Please make an individual pull request for each suggestion.
+
+4. Please use the following format for libraries: \[LIBRARY\]\(LINK\) - DESCRIPTION.
+
+5. Please prefix duplicate library names with their vendor or namespace followed by a space: Orno\DI would be Orno DI.
+
+6. Please keep descriptions short and simple. 
+
+7. Please end all descriptions with a full stop/period.
+
+8. Please check your spelling and grammar.
+
+I try to open a suggestions issue each month, so feel free to add anything interesting to it. I’m also open to creating new categories or improving the existing ones.
+
+Thank you for your suggestions!