Consider adding `pipx` to install docs

[pipx](https://pypa.github.io/pipx/) is a tool that allows you to install python packages into isolated environments. 

It ensures that there aren't any package management issues, and allows you to run the packages from the command line always in their isolated environments. 

It's how I've been using `solc-select`.

```bash
pipx install solc-select
which solc
which solc-select
``` 