Update 'NixOS installation'

NixOS-installation.md

@@ -1,4 +1,4 @@
-
+I used [this guide](https://nixos.wiki/wiki/Wil_T_Nix_Guides) so it should be straightforward for you too.
 
 ```
 < Connect to the Internet />
@@ -16,3 +16,16 @@ mkdir -p /mnt/boot/efi
 mount /dev/disk/by-label/NIXBOOT /mnt/boot/efi
 swapon /dev/disk/by-label/SWAP
 ```
+
+```
+git init
+git crypt init
+git crypt add-gpg-user <email>
+git crypt export-key <out-path>
+```
+
+Add a `.gitattributes` file containing
+
+```
+secrets/** filter=git-crypt diff=git-crypt
+```