Added external.ts file and reference to it in the README.
Added external.ts file and reference to it in the README.

FluxDZ/PythonEditor,jamesblissett/PythonEditor,bbcmicrobit/PythonEditor,jamesblissett/PythonEditor,ben-dent/PythonEditor,ben-dent/PythonEditor,jamesblissett/PythonEditor,ben-dent/PythonEditor,FluxDZ/PythonEditor,bbcmicrobit/PythonEditor,bbcmicrobit/PythonEditor,FluxDZ/PythonEditor,bbcmicrobit/PythonEditor