Android install tutorial in README.md

[Usinganame]

Add a tutorial for installing on Android/Termux

As of now, fish works perfectly under Android/Termux. All that you need to do it run
pkg install fish after installing Termux

[Fabian Boehm]

We don't list every single distribution in the README.

In general I believe that people can just check their package manager - it's the obvious way to install something.