Add a note on free tier

.github/CONTRIBUTING.md

@@ -4,6 +4,8 @@
 opened to market company APIs that offer paid solutions. This API list is not a marketing tool, but a tool to help the
 community build applications and use free, public APIs quickly and easily. Pull requests that are identified as marketing attempts will not be accepted.
 >
+> Please make sure the API you want to add has a full free access or at least a free tier before submitting.
+>
 > Thanks for understanding! :)
 
 ## Formatting