Update relase-notes.txt for StartImmediateAsTask performance improvement

Author: abelbraaksma

release-notes.txt

@@ -4,6 +4,7 @@ Release notes:
     - overhaul all doc comments, add exceptions info, improve tooltips experience, #136
     - DEPRECATED: type 'TaskSeq<'T>' now replaces 'taskSeq<'T>', the latter deprecated FS0044, #187
     - BINARY INCOMPATIBLE: TaskSeq module members are now static members, source code compatible, #186
+    - Performance: use StartImmediateAsTask instead of StartAsTask to prevent thread-hops, #135
 
 0.4.0-alpha.1
     - fixes not calling Dispose for 'use!', 'use', or `finally` blocks #157 (by @bartelink)