chore: update docs to mention Nixpkgs and xzp support

Author: craftablescience

INSTALL.md

@@ -67,3 +67,10 @@ There are two ways of *installing* VPKEdit specific to the following distros:
 VPKEdit is on the Arch User Repository thanks to [@HurricanePootis](https://github.com/HurricanePootis).
 Please consult the [Arch Wiki](https://wiki.archlinux.org/title/Arch_User_Repository) for more information,
 as I don't use Arch Linux personally and don't know how that system works. (I use Debian btw)
+
+#### NixOS:
+
+VPKEdit is available from Nixpkgs thanks to [@Seraphim Pardee](https://github.com/SeraphimRP).
+To install to the default profile, run `nix-env -i vpkedit`. See the 
+[Nix documentation](https://nix-tutorial.gitlabpages.inria.fr/nix-tutorial/getting-started.html)
+for more information.

README.md

@@ -50,6 +50,7 @@ Please read [this file](https://github.com/craftablescience/VPKEdit/blob/main/IN
   | VPK      | Vampire PacK file (V:TMB)                         |   ✅    |  ✅   |   ✅   |
   | VPP      | Volition Pack file (Red Faction, Saints Row)      |   ❌    |  ✅   |   ❌   |
   | WAD      | Where's All the Data (GoldSrc Engine)             |   ✅    |  ✅   |   ✅   |
+  | XZP      | Xbox ZiP (Xbox Half-Life 2)                       |   ❌    |  ✅   |   ❌   |
   | ZIP      | ZIP file                                          |   ✅    |  ✅   |   ✅   |
 
 - Preview files contained within the pack file without extracting them:
@@ -68,7 +69,8 @@ Please read [this file](https://github.com/craftablescience/VPKEdit/blob/main/IN
   - Directly edit text files within writable pack files
 - Extract files and folders from readable pack files
 - Switch version of existing VPKs
-- Native Linux compatibility
+- Native Linux builds
+  - Packaged for Debian, Arch, NixOS; standalone builds available
 - User interface translated to the following languages ([support the translation effort here](https://poeditor.com/join/project/yxR9MLc9X2)):<br/>
   🇧🇦 🇨🇳 🇭🇷 🇳🇱 🇩🇪 🇮🇹 🇯🇵 🇰🇷 🇵🇱 🇧🇷 🇷🇺 🇪🇸 🇸🇪 🇻🇳