Add contribution guidelines

CONTRIBUTING.md

@@ -0,0 +1,18 @@
+# Contribution Guidelines
+
+Please ensure your pull request adheres to the following:
+
+* Search previous suggestions before making a new one to ensure yours is not a duplicate.
+* Make an individual pull request for each suggestion.
+* Use the following format: \[NAME\]\(LINK\) - DESCRIPTION.
+* Keep descriptions simple.
+* New categories, or improvements to the existing ones are welcome.
+* End all descriptions with a period.
+* Make sure your text editor is set to remove trailing whitespace.
+* Try to ensure your pull requests only have one commit if possible. You can squash your commits into one before submitting.
+* Make your pull request titles and descriptions as detailed as possible. Explain why an asset or resources deserves to be added to the list.
+* Make sure your additions are in alphabetical order. This includes the headers and items themselves.
+* Ensure that assets you submit are of high quality, documented well, and are recently maintained. It also helps if they're popular so I can research them before adding them to the list.
+* This is not meant to be used as a list to advertise your own assets. Free or open-source assets should be prioritized over paid assets if they are of similar quality and feature sets.
+
+Thanks!