Plane: update release notes for 3.9.8

ArduPlane/release-notes.txt

@@ -1,5 +1,5 @@
-Release 3.9.8-beta1, 22nd April 2019
+Release 3.9.8, 26th April 2019
-------------------------------------
+------------------------------
 
 This is a minor release with some important safety fixes. The fixes
 relate to two issues:
@@ -68,6 +68,11 @@ Other changes in this release:
  - fixed fallback to microSD for parameter storage if a board with
    FRAM storage has a failed FRAM device
 
+ - added a specific check for hw failure regarding the recent
+   CubeBlack safety bulletin
+
+ - fixed the power flags for brick2 on fmuv5
+
 Happy flying!