Update README.md

Author: init-ok

README.md

@@ -7,6 +7,8 @@ This script allows you to export a .OBJ file from a Sketchfab model by clicking
 
 Installation:  
   You must install the userscript to see the 'Download .OBJ' button on the model viewer page.  
+  To do this, you must have a userscript manager installed.  
+  If you have one installed, clicking on [this link](https://github.com/reinitialized/sketchfab-dl-script/raw/master/sketchfab-dl-script.user.js) should bring up a prompt to install the script.  
   I have tested the script so far on Tampermonkey (Chrome) and Greasemonkey (Firefox).  
 
 Todo: