Add pull request trigger info for Ubuntu 20.04, CentOS 8 and Amazon Linux 2

Author: shahmishal

docs/ContinuousIntegration.md

@@ -92,6 +92,9 @@ macOS platform               | @swift-ci Please smoke benchmark              | S
 Linux platform               | @swift-ci Please test Linux platform          | Swift Test Linux Platform (smoke test)<br>Swift Test Linux Platform
 Linux platform               | @swift-ci Please clean test Linux platform    | Swift Test Linux Platform (smoke test)<br>Swift Test Linux Platform
 macOS platform               | @swift-ci Please ASAN test                    | Swift ASAN Test OS X Platform
+Ubuntu 20.04                | @swift-ci Please test Ubuntu 20.04 platform    | Swift Test Ubuntu 20.04 Platform
+CentOS 8                    | @swift-ci Please test CentOS 8 platform    | Swift Test CentOS 8 Platform
+Amazon Linux 2              | @swift-ci Please test Amazon Linux 2 platform    | Swift Test Amazon Linux 2 Platform
 
 The core principles of validation testing is that: