CHANGELOG location is inconsistent with HDF5

[mattjala]

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

[mattjala]

A decision has been reached on this - remove it from the User's guide and move it to the docs folder.