docs: introduce document to announce breaking changes

Over time, Git has grown quite a lot. With this evolution, many ideas
that were sensible at the time they were introduced are not anymore and
are thus considered to be deprecated. And while some deprecations may be
noted in manpages, most of them are actually deprecated in the "hive
mind" of the Git community, only.

Introduce a new document that tracks such breaking changes, but also
deprecations which we are not willing to go through with, to address
this issue. This document serves multiple purposes:

  - It is a way to facilitate discussion around proposed deprecations.

  - It allows users to learn about deprecations and speak up in case
    they have good reasons why a certain feature should not be
    deprecated.

  - It states intent and documents where the Git project wants to go,
    both in the case where we want to deprecate, but also in the case
    where we don't want to deprecate a specific feature.

The document is _not_ intended to cast every single discussion into
stone. It is supposed to be a living document that may change over time
when there are good reasons for it to change.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Documentation/BreakingChanges.txt b/Documentation/BreakingChanges.txt
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+= Upcoming breaking changes
+
+The Git project aims to ensure backwards compatibility to the best extent
+possible. Minor releases will not break backwards compatibility unless there is
+a very strong reason to do so, like for example a security vulnerability.
+
+Regardless of that, due to the age of the Git project, it is only natural to
+accumulate a backlog of backwards-incompatible changes that will eventually be
+required to keep the project aligned with a changing world. These changes fall
+into several categories:
+
+* Changes to long established defaults.
+* Concepts that have been replaced with a superior design.
+* Concepts, commands, configuration or options that have been lacking in major
+  ways and that cannot be fixed and which will thus be removed without any
+  replacement.
+
+Explicitly not included in this list are fixes to minor bugs that may cause a
+change in user-visible behavior.
+
+The Git project irregularly releases breaking versions that deliberately break
+backwards compatibility with older versions. This is done to ensure that Git
+remains relevant, safe and maintainable going forward. The release cadence of
+breaking versions is typically measured in multiple years. We had the following
+major breaking releases in the past:
+
+* Git 1.6.0, released in August 2008.
+* Git 2.0, released in May 2014.
+
+We use <major>.<minor> release numbers these days, starting from Git 2.0. For
+future releases, our plan is to increment <major> in the release number when we
+make the next breaking release. Before Git 2.0, the release numbers were
+1.<major>.<minor> with the intention to increment <major> for "usual" breaking
+releases, reserving the jump to Git 2.0 for really large backward-compatibility
+breaking changes.
+
+The intent of this document is to track upcoming deprecations for future
+breaking releases. Furthermore, this document also tracks what will _not_ be
+deprecated. This is done such that the outcome of discussions document both
+when the discussion favors deprecation, but also when it rejects a deprecation.
+
+Items should have a clear summary of the reasons why we do or do not want to
+make the described change that can be easily understood without having to read
+the mailing list discussions. If there are alternatives to the changed feature,
+those alternatives should be pointed out to our users.
+
+All items should be accompanied by references to relevant mailing list threads
+where the deprecation was discussed. These references use message-IDs, which
+can visited via
+
+  https://lore.kernel.org/git/$message_id/
+
+to see the message and its surrounding discussion. Such a reference is there to
+make it easier for you to find how the project reached consensus on the
+described item back then.
+
+This is a living document as the environment surrounding the project changes
+over time. If circumstances change, an earlier decision to deprecate or change
+something may need to be revisited from time to time. So do not take items on
+this list to mean "it is settled, do not waste our time bringing it up again".
+
+== Git 3.0
+
+The following subsections document upcoming breaking changes for Git 3.0. There
+is no planned release date for this breaking version yet.
+
+Proposed changes and removals only include items which are "ready" to be done.
+In other words, this is not supposed to be a wishlist of features that should
+be changed to or replaced in case the alternative was implemented already.
+
+=== Changes
+
+=== Removals
+
+== Superseded features that will not be deprecated
+
+Some features have gained newer replacements that aim to improve the design in
+certain ways. The fact that there is a replacement does not automatically mean
+that the old way of doing things will eventually be removed. This section tracks
+those features with newer alternatives.