Added nixos-install instructions

NixOS-installation.md

@@ -16,6 +16,9 @@ mount /dev/disk/by-label/NIXBOOT /mnt/boot/efi
 swapon /dev/disk/by-label/SWAP
 ```
 
+<details>
+    <summary>If you do not have an existing configuration repository.</summary>
+	
 ```
 git init
 git crypt init
@@ -27,4 +30,22 @@ Add a `.gitattributes` file containing
 
 ```
 secrets/** filter=git-crypt diff=git-crypt
-```
+```
+    
+</details>
+<details>
+    <summary>If you have a repository and you want to 'clone' an existing machine using flakes.</summary>
+    
+Clone your repository and go inside the folder containing the `flake.nix` file.
+    
+```
+nixos-install --flake .#<hostname>
+```
+    
+Alternatively, if you don't want to clone the repository first:
+    
+```
+nixos-install --flake https://<url_to_your_repo>#<hostname>
+```
+    
+</details>