add instructions to update mvt via pipx

[carlosm2]

when an update is available on mvt:

    MVT - Mobile Verification Toolkit

        https://mvt.re
        Version: 2.5.4
        Version 2.6.0 is available! Upgrade mvt with `pip3 install -U mvt`

There is an instruction to upgrade: 'Upgrade MVT with pip3 install -U mvt.' This is useful, but for those who installed mvt via pipx, additional instructions could be included.

'pipx upgrade mvt' in case you installed via pipx

[roaree]

Thanks for the catch!

[carlosm2]

I've just added it. I hope it's clear and useful.
728d472

[besendorf]

This can be closed then, right? @carlosm2

[carlosm2]

yes, thanks