chore: minor documentation fix

README.md

@@ -189,7 +189,7 @@ var defaultOptions = showdown.getDefaultOptions();
 
  * **noHeaderId**: (boolean) [default false] Disable the automatic generation of header ids. Setting to true overrides **prefixHeaderId**
 
- * **ghCompatibleHeaderId**: (boolean) [default false] Generate header ids compatible with github style (spaces are replaced with dashes, ][&~$!@#*()=:/,;?+'.%\ chars are removed
+ * **ghCompatibleHeaderId**: (boolean) [default false] Generate header ids compatible with github style (spaces are replaced with dashes, ][&~$!@#*()=:/,;?+'.%\ chars are removed) (since v1.5.5)
 
  * **prefixHeaderId**: (string/boolean) [default false] Add a prefix to the generated header ids. Passing a string will prefix that string to the header id. Setting to `true` will add a generic 'section' prefix.