Update release notes for 1.15
Update release notes for 1.15
Merge branch 'wm-notes-1.15.0' of https://github.com/rethinkdb/rethinkdb into wm-notes-1.15.0

Conflicts:
	NOTES.md
Add missing PR
Add a few newly-closed issues
Fix version number
Fix errant edit
Updates based on comments
More updates, two new issues
Change release name
Change 'update' to 'replace'
Clarify "upgrading" tables
Add note about #3087
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Merge branch wm-notes-1.15.0 into v1.15.x