Mention Ubuntu 20.04 package

Ubuntu 20.04 has been released today, and scrcpy is available in their
repositories: <https://packages.ubuntu.com/focal/scrcpy>

README.md

@@ -37,7 +37,7 @@ control it using keyboard and mouse.
 
 ### Linux
 
-In Debian (_testing_ and _sid_ for now):
+On Debian (_testing_ and _sid_ for now) and Ubuntu (20.04):
 
 ```
 apt install scrcpy