Update README.md

Author: mevdschee

README.md

@@ -22,6 +22,7 @@ This is a single file application! Upload "api.php" somewhere and enjoy!
 
 ## Limitations
 
+  - Primary keys should either be auto-increment (from 1) or UUID
   - Composite primary or foreign keys are not supported
   - Complex filters (with both "and" & "or") are not supported
   - Complex writes (transactions) are not supported