DOC: add note about many removals in pandas 1.0
cc @TomAugspurger 
Adding a note about the recommended way to upgrade to pandas 1.0, if that sounds OK
(this has always been the case before with our rolling deprecations, but I think it can be stressed now more because 1) we removed really a lot and 2) we also removed things only deprecated in 0.25) Looks good, can you fix the merge conflict? @TomAugspurger : Merge conflict in this PR was introduced by me, so I resolved it myself.

(an aside: surprised that there was a merge conflict.  The diff in no way overlaps)