updating-experimental.txt

The `experimental` branch contains changes that are required for
yet unreleased versions of Agda. These are kept separate from
`master` so that the standard library releases can occur independently
from Agda releases.

To update `experimental` to the current version of `master` run the
following:
  ```
  git checkout master
  git pull
  git checkout experimental
  git merge master
  git push
  ```