add a reminder to the contributing guideline

Author: vinta

CONTRIBUTING.md

@@ -2,13 +2,13 @@
 
 Your contributions are always welcome!
 
-## Guidelines
+## 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 library is awesome.
 * Add the link: `* [project-name](http://example.com/) - A short description ends with a period.`
-    * Keep descriptions concise.
+    * Keep descriptions concise and **short**.
 * Add a section if needed.
     * Add the section description.
     * Add the section title to Table of Contents.
@@ -17,3 +17,4 @@ Your contributions are always welcome!
 * Check your spelling and grammar.
 * Remove any trailing whitespace.
 
+Just a gentle reminder: **Try not to submit your own project. Instead, waiting for someone finds it useful and submit it for you.**