Document how to build and install this

Document how to build and install this for non-Rust regulars.
Tested on Debian GNU/Linux 12 (bookworm).

```
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
. "$HOME/.cargo/env" 
git clone https://github.com/linux-packaging-rs/flatpak-ext-tools
cd flatpak-ext-tools

sudo apt-get -y install libssl-dev libflatpak-dev

cargo build

# target/debug/flatpak-ext
```