[TD-6420]<docs>: update README.md for apt-get install.

README.md

@@ -202,6 +202,16 @@ taos
 
 If TDengine shell connects the server successfully, welcome messages and version info are printed. Otherwise, an error message is shown.
 
+## Install TDengine by apt-get
+```
+wget -qO - http://repos.taosdata.com/tdengine.key | sudo apt-key add -
+echo "deb [arch=amd64] http://repos.taosdata.com/tdengine-stable stable main" | sudo tee /etc/apt/sources.list.d/tdengine-stable.list
+[Optional] echo "deb [arch=amd64] http://repos.taosdata.com/tdengine-beta beta main" | sudo tee /etc/apt/sources.list.d/tdengine-beta.list
+sudo apt update
+apt policy tdengine
+sudo apt install tdengine
+```
+
 ## Quick Run
 
 If you don't want to run TDengine as a service, you can run it in current shell. For example, to quickly start a TDengine server after building, run the command below in terminal: (We take Linux as an example, command on Windows will be `taosd.exe`)