CHANGELOG location is inconsistent with HDF5

Ideally, the CHANGELOG.md file would be in `docs` instead of `docs/UserGuide` for consistency with its placement in the HDF5. However, in order to include it in the User Guide, it had to be under the `UserGuide` directory.

One of the following should be done before the next release:
- Move the CHANGELOG to the higher level directory, and remove it from the User Guide
- Move the CHANGELOG to the higher level directory, and somehow embed its contents/a link to it within the User Guide
- Leave the CHANGELOG in its current location