Add head option to readme

Author: Dineshs91

Readme.md

@@ -33,6 +33,7 @@ These are keys in the options object you can pass to the progress bar along with
 - `total` total number of ticks to complete
 - `width` the displayed width of the progress bar defaulting to total
 - `stream` the output stream defaulting to stderr
+- `head` head character defaulting to complete character
 - `complete` completion character defaulting to "="
 - `incomplete` incomplete character defaulting to "-"
 - `renderThrottle` minimum time between updates in milliseconds defaulting to 16