GHC boot-library package changelogs & release-notes

[Herbert Valerio Riedel]

Hello fellow GHC devs,

As some of you might have noticed, I added a changelog.md file to
libraries/base:

 https://github.com/ghc/packages-base/blob/c8634027d4e3315a2276fb1be8168c486419785a/changelog.md

(please feel free to fix any typos/omissions/whatever you notice)

My hope/motivation is that since Hackage gained the ability to display
changelog files, the rather extensive changes in `base` might be a bit
more easily/conveniently accessible on Hackage.

I chose to use Markdown format, as I believe it may be more convenient
to maintain/edit the `base` changelog as plain text rather than having
to edit XML after each noteworthy change in `base`. And as the
release-notes typically only exploit a subset of the Docbook facilities,
the conversion to Docbook XML could be performed semi-automatically
shortly before a release.

Moreover, the release notes from previous major GHC release (which in
the past contained the major changes in `base` et al.) are usually
removed again. While a separate changelog file would usually retain
(more) version history.

Therefore, I'd propose to switch from editing the user's guide release
note for library release notes to using Hackage-changelog files in
Markdown format (following a common structural convention) and make it
the release-manager's responsibility to integrate the respective
package's changelog content into the user's guide.

Any comments?

Cheers,
  hvr