DOC : added folders for api_changes and whats_new

Added README + folders for whats_new and api_changes files.

The idea is instead of everyone editing the same file (which leads to
frequent conflicts requiring either manual merging or re-basing) each
entry will create a new file which will be assembled into a single file
during the release.
DOC : tweaked api_change/whats_new README

 - fixed typos
 - changed txt -> rst so gh will display it nicely (maybe?)
 - minor prose change to be clearer
Merge pull request #3349 from tacaswell/api_whats_new_folders

DOC : added folders for api_changes and whats_new