nixos/filesystems: add overlayfs docs

nixos/doc/manual/configuration/file-systems.chapter.md

@@ -39,4 +39,5 @@ and non-critical by adding `options = [ "nofail" ];`.
 ```{=include=} sections
 luks-file-systems.section.md
 sshfs-file-systems.section.md
+overlayfs.section.md
 ```

nixos/doc/manual/configuration/overlayfs.section.md

@@ -0,0 +1,27 @@
+# Overlayfs {#sec-overlayfs}
+
+NixOS offers a convenient abstraction to create both read-only as well writable
+overlays.
+
+```nix
+fileSystems = {
+  "/writable-overlay" = {
+    overlay = {
+      lowerdir = [ writableOverlayLowerdir ];
+      upperdir = "/.rw-writable-overlay/upper";
+      workdir = "/.rw-writable-overlay/work";
+    };
+    # Mount the writable overlay in the initrd.
+    neededForBoot = true;
+  };
+  "/readonly-overlay".overlay.lowerdir = [
+    writableOverlayLowerdir
+    writableOverlayLowerdir2
+  ];
+};
+```
+
+If `upperdir` and `workdir` are not null, they will be created before the
+overlay is mounted.
+
+To mount an overlay as read-only, you need to provide at least two `lowerdir`s.