Update README.md

Make it extra clear to modify response headers rather than request headers - I spent 5 mins trying to figure out what was wrong and would rather save someone the time.

Author: cstavitsky

README.md

@@ -27,7 +27,7 @@ note: a .env file must be made in order to store the relative variable value pro
 ![Alt Text](performance.gif)
 
 ## Enable Profiling
-In order to enable profiling, please use an extension that allows you to modify the document response headers. An example of an extension like that for Chrome is [modheader](https://modheader.com/).
+In order to enable profiling, please use an extension that allows you to modify the document response headers (make sure you create a rule to modify RESPONSE headers, not request headers!). An example of an extension like that for Chrome is [modheader](https://modheader.com/).
 
 ![policy headers](profiling.png)