Update url's in Readme

Mention support for encrypted exe's in -ng version

Mention support for 5.2, 5.3

Correctly calculate the python version, fixes extremecoders-re#37

The python version is stored as major*100 + minor

Details: pyinstaller/pyinstaller@9f7c92

 Mention support for pyinstaller 4.10, 5.0, 5.0.1

 Mention support for pyinstaller 4.7