Update readme.md

Author: beanieboi

readme.md

@@ -14,6 +14,8 @@ Some application servers (e.g. Ruby's Unicorn) halt progress when dealing with n
 * NGINX Version: 1.9.5
 ### Heroku 18
 * NGINX Version: 1.18.0
+### Heroku 20
+* NGINX Version: 1.18.0
 
 ## Requirements (Proxy Mode)