Remove changelog and document automated creation (Qiskit#3109)

* Remove changelog and document automated creation

With the introduction of the qiskit-bot release automation for terra we
no longer need to manually maintain or keep a changelog in repo. The
github release pages (which we were manually copy and pasting the
changelog into already) take the place of this file. For all future
releases the creation of this page is completely automatic and handled
by the bot. This works by using the git log and PR tags to figure out
which commits belong in the changelog summary. The commit msg summary
(the first line) is used for the changelog based on the tags. If a
longer explanation is necessary that should be handled in the reno
release notes.

Fixes Qiskit#3077

* Add README section on changelog

* Make tweaks to readme for clarity