Add the next/prev commands to the RELEASENOTES file

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>
diff --git a/RELEASENOTES b/RELEASENOTES
index f6a8b85..0a220a4 100644
--- a/RELEASENOTES
+++ b/RELEASENOTES
@@ -31,6 +31,9 @@
       + "stg publish", for maintaining merge-friendly branches (which
         are not rebased).
 
+      + "stg prev/next" for printing the name of the previous or next
+        patch in the series.
+
   - The commands "stg add", "stg rm", "stg cp", and "stg resolved"
     were removed, since there are corresponding Git equivalents