Add info to install libappindicator

Author: mastrolinux

README.md

@@ -34,7 +34,7 @@ will build the `arduino-create-agent` binary.
 You can use `bootstrapPlatforms` function to compile the needed CGO-enabled environment
 
 Other prerequisites are:
-* libappindicator (Linux only)
+* libappindicator (Linux only on Ubuntu `sudo apt-get install libappindicator1`)
 * [go-selfupdate] (https://github.com/sanbornm/go-selfupdate) if you want to test automatic updates
 
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