README: Add a note for p4a commnad not found

The python-for-android providing "p4a" command is installed from the
requirements.txt in this repository. The installation is bounded in the
user enviroment by default, not global.

However, not every distribution's environment variable "PATH" includes
the user's HOME path "~/.local/bin". That leads the "p4a" command in
"~/.local/bin" becoming missed. So, add this note directing to the
solution: "p4a: command not found" [1].

[1]: kivy/python-for-android#1167

README.md

@@ -38,6 +38,8 @@ To download a Kolibri WHL file, you can use `make whl=<URL>` from the command li
 
 6. Run `make kolibri.apk.unsigned` to build the apk. Watch for success at the end, or errors, which might indicate missing build dependencies or build errors. If successful, there should be an APK in the `dist/` directory.
 
+PS. If `p4a` command is not found, please check the ticket: ["p4a: command not found"](https://github.com/kivy/python-for-android/issues/1167)
+
 ## Build on Toolbox
 
 Toolbox allows a mixture of the above build processes by providing a