ref-manual: document overlayfs-etc image feature

(From yocto-docs rev: 62c7cea1b916c04bfd5c25e0011424b90d5272a1)

Signed-off-by: Vyacheslav Yurkov <uvv.mail@gmail.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/features.rst

@@ -219,6 +219,10 @@ Here are the image features available for all images:
 -  *empty-root-password:* Sets the root password to an empty string,
    which allows logins with a blank password.
 
+-  *overlayfs-etc:* Configures the ``/etc`` directory to be in ``overlayfs``.
+   This allows to store device specific information elsewhere, especially
+   if the root filesystem is configured to be read-only.
+
 -  *package-management:* Installs package management tools and preserves
    the package manager database.