releases: update to include 3.1.16

(From yocto-docs rev: e33a626774034a833a92108c2adf6799098685c0)

Signed-off-by: Michael Halstead <mhalstead@linuxfoundation.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/releases.rst

@@ -58,6 +58,7 @@ Release Series 3.1 (dunfell)
 - :yocto_docs:`3.1.13 Documentation </3.1.13>`
 - :yocto_docs:`3.1.14 Documentation </3.1.14>`
 - :yocto_docs:`3.1.15 Documentation </3.1.15>`
+- :yocto_docs:`3.1.16 Documentation </3.1.16>`
 
 ==========================
  Outdated Release Manuals