Things we'll want to put forward in the release notes (feel free to complete):

• removed Global Set SsrOldRewriteGoalsOrder in ssreflect.v, for a
  smooth transition, we recommend adding
  Set SsrOldRewriteGoalsOrder. (* change Set to Unset when porting the file, then remove the line when requiring MathComp >= 2.6 *)
  in each of your files after requiring ssreflect.v (even
  indirectly), which will enable porting to the new rewrite subgoals
  order on a file per file basis
• new files ring_tactic.v, field_tactic.v and
  arithmetic_tactic.v in the mathcomp-algebra package, offering
  decision procedures ring, field and lra respectively (formerly
  available in the mathcomp-algebra-tactics package)
• new file tensor.v, this remains experimental and may change
  without deprecation warning in the next release
• packages fingroup and character have been renamed to
  finite-group and group-representation respectively
• many generalization, file splitting and cleanup, these should
  ideally remain transparent to users or at worst easy to adapt to,
  following the deprecation messages