Update CHANGES.rst

These changelog entries were in the wrong section, as confirmed by Marten.
Merge pull request #1933 from mdboom/unit/less-did-you-mean

Creating TimeDeltas is now extremely slow
Merge pull request #1918 from mdboom/wcs/wcsprm-members

WCS changes velocity units from km/s to m/s
Accept multiple slashes in unit strings.
Accept "angstrom" (lower-case a) as a deprecated unit in FITS
Skip some doctests, since we can't reliably always get a warning out.
Fix doctest for upstream change.