migration-guides: add release notes for 5.1.1

(From yocto-docs rev: db2ff7bbe64e1dca6b0868046f478d7622f79865)

Signed-off-by: Lee Chee Yang <chee.yang.lee@intel.com>
Reviewed-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/migration-guides/release-5.1.rst

@@ -7,3 +7,4 @@ Release 5.1 (styhead)
 
    migration-5.1
    release-notes-5.1
+   release-notes-5.1.1