vcmi.eu/players/Installation_Linux: please mention Gentoo ebuild

https://vcmi.eu/players/Installation_Linux/

Please add [gamerlay](https://github.com/gentoo-mirror/gamerlay/tree/master/games-strategy/vcmi) overlay:

```bash
eselect repository enable gamerlay
emaint sync -r gamerlay
```

Add to `/etc/portage/package.accept_keywords`

```
=games-strategy/vcmi-9999 **
	dev-libs/fuzzylite ~amd64
```

and finally

```bash
emerge -av vcmi
```