Add helios configuration

hosts/helios/README.md

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+# Helios
+
+## Overview
+Dell OptiPlex 980 small form factor desktop.
+
+## Specs
+* CPU - Intel Core i7-860 @ 2.8GHz
+* Memory - 8 GB DDR3
+* GPU - AMD FirePro 2460
+
+### Disks
+Device | Partitions _(filesystem, usage)_
+--- | ---
+SanDisk Ultra II | `/dev/sda1` (ext2, NixOS Boot) <br> `/dev/sda2` (ZFS, NixOS Root)
+
+#### ZFS pool layout
+```
+rpool/
+├── local
+│   ├── nix
+│   └── tmp
+├── system
+│   ├── root
+│   └── var
+└── user
+    └── home
+```
+
+See [Graham Christensen's article](https://grahamc.com/blog/nixos-on-zfs/#datasets) for the motivation behind these datasets.
+
+### Networks
+- DHCP on `192.168.0.0/32` subnet.
+- Tailscale on `100.64.0.0/10` subnet. FQDN: `helios.mesh.vimium.net`.