Changelog: Warn about upcoming breaking changes

[sjakobi]

One concern that frequently came up in discussions over breaking changes is that many packages currently don't specify an upper bound on bytestring. The note I'm adding here is meant to help remediate that situation.

The planned breaking changes I'm currently aware of are:

• Starting offset not necessary #174
• findSubstrings is broken #180

and potentially the result of #140.

[sjakobi]

Suggestions to improve the wording of the note are very welcome! :)

[Bodigrim]

It does not hurt, but basically reiterates the fact that major versions bring breaking changes. If some people prefer not to put upper bounds beforehand, I do not think that such warning itself (even if read) will convince them to do otherwise, to be honest.

To be a good citizen, I'd probably try to build a Stackage snapshot, once breaking changes have landed, and report failures to respective maintainers.

[sjakobi]

@Bodigrim bytestring is a bit of a special case in that its last major version bump was 8 years ago! Most Haskell users have probably only ever used some v0.10.*. So I think this reminder could be helpful, even if it won't reach everyone, and not everyone who reads it will act upon it.

Regarding smoke-testing breaking changes, I've inquired about how to do this before, but never actually ended up doing it:

• https://gitter.im/commercialhaskell/stackage?at=5df717e7ce2b2e652be38b5a
• Can't print dependencies of bytestring cdepillabout/nix-reverse-deps-of-haskell-package#1

Luckily, once we've released a breaking change, the Stackage maintainers will eventually take of smoke-testing! ;)

[Bodigrim]

Wording looks good to me :)

[sjakobi]

@hsyl20, @bgamari Could you review this too? :)

[hsyl20]

LGTM :)

[dcoutts]

LGTM!