System Images: Recommend "plain" images as well

"plain" is what we use for our production images. Right now those
two types are mutually exclusive (image kinds that offer "plain"
don't offer "current" and vice versa), so let's mention both to avoid
confusion.

Contact/System_Images.rst

@@ -45,6 +45,6 @@ This indicates the stability of the kernel used in the image:
  * ``legacy``: an old kernel, kept around to diagnose regressions
  * ``plain``: the plain kernel from the Debian/PureOS archive
 
-Generally, users looking for a stable experience should use images with ``current`` kernels. Those looking to fix problems with the kernel itself will probably use images with ``next`` kernels.
+Generally, users looking for a stable experience should use images with ``plain`` or ``current`` kernels. Those looking to fix problems with the kernel itself will probably use images with ``next`` kernels.
 
 .. include:: /links.txt