add a line about contributing

README.md

@@ -19,6 +19,10 @@ You can slow it down using the `-C` argument, or using sleep instructions.
 
 Maybe this is not for you
 
+### Contributing
+
+Yup, go ahead. You can also develop your own private *crsn* extensions, they work like plugins.
+
 # Architecture
 
 The runtime is built as a register machine with a stack and status flags.