diff --git a/README.md b/README.md
index 9d0ed34..2661434 100644
--- a/README.md
+++ b/README.md
@@ -89,6 +89,7 @@ $ sudo apt install gnupg2
 $ gpg2 --keyserver hkp://keys.gnupg.net --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3 7D2BAF1CF37B13E2069D6956105BD0E739499BDB
 $ \curl -sSL https://get.rvm.io | bash -s stable
 $ source /home/rulingcom/.rvm/scripts/rvm
+# $ sudo sudo vi /etc/apt/sources.list # Ubuntu 20.04LTS add `deb http://security.ubuntu.com/ubuntu bionic-security main`
 $ rvm install 2.1.2
 $ rvm install 2.1.10
 $ rvm --default use 2.1.10