message: [skip ci] Add exact betti curve in release note
diff: @@ -6,8 +6,8 @@ We are now using GitHub to develop the GUDHI library, do not hesitate to [fork t
  Below is a list of changes made since GUDHI 3.5.0:
  -- [Module](link)
  - - ...
  +- [Representations](https://gudhi.inria.fr/python/latest/representations.html#gudhi.representations.vector_methods.BettiCurve)
  + - A more flexible Betti curve class capable of computing exact curves
  - [Module](link)
  - ...
  