add note to 'Contributing' chapter

- signed-off-by: trimstray <trimstray@gmail.com>

README.md

@@ -66,6 +66,10 @@ These below rules may be better:
 - easy to contribute to (Markdown + HTML ...)
 - easy to find (simple TOC, maybe it's worth extending them?)
 
+Remember this when you want to add something new:
+
+  > **_This repository is not meant to contain everything that is available - only good quality stuff._**
+
 Url marked **\*** is temporary unavailable. Please don't delete it without confirming that it has permanently expired.
 
 Before adding a pull request, please see the **[contributing guidelines](CONTRIBUTING.md)**. All **suggestions/PR** are welcome!