Update contributing section

Author: Joy-less

README.md

@@ -637,7 +637,7 @@ Contributions are welcome.
 
 ### Suggestions
 
-Please raise an issue. Note that the format shouldn't stray too far from HJSON or JSON5 and should be directly translatable to JSON.
+Since the language is now stable, language suggestions are unlikely to be accepted. If you would like to suggest a change anyway, please raise an issue. Note that the format shouldn't stray too far from HJSON or JSON5, and must be directly translatable to JSON.
 
 ### Implementations