Update readme.md

Author: ajaymare

readme.md

@@ -69,6 +69,7 @@
 [Remove X-* and Server Headers from Response](security/remove_x_headers_from_http_resp.md)  
 [Close Connections without Host header](security/close_connection_without_host.md)  
 [Redirect	Location	header	validator](security/redirect_location_header_validator.md)
+[Let's Encrypt Server chanllange response](security/let's_Encrypt_Response.md)
 
 ## Acceleration
 [Client Cache Control Behavior](availability/client_cache_control_behavior.md)