"Since" information for `MonadFail` instances missing (haddock, CHANGELOG)

[andreasabel]

After doing a bit of research, I conjecture that the MonadFail instances (e.g. for ReaderT) were added in mtl-2.2.1.
However, this information isn't attached to these instances in the haddocks (e.g. https://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Reader.html#v:fail), nor is it mentioned in the CHANGELOG (https://hackage.haskell.org/package/mtl-2.2.2/changelog).

Proposal:

• add missing since information to the docs
• also add to CHANGELOG

[sjshuck]

Proposal accepted 🙂