README: add Guix System to installation table (#615)

Author: RishikesavanRamesh

README.md

@@ -91,6 +91,12 @@ $ age --decrypt -i key.txt data.tar.gz.age > data.tar.gz
             <code>emerge app-crypt/age</code>
         </td>
     </tr>
+    <tr>
+        <td>Guix System</td>
+        <td>
+            <code>guix package -i age</code>
+        </td>
+    </tr>
     <tr>
         <td>NixOS / Nix</td>
         <td>