Add Breaking Changes section to docs

Created a new `breaking_changes.rst` file to document changes incompatible with
earlier versions. The impetus for this is the beginning of the long overdue
refactoring of the repository to remove old cruft and modernize its packaging
and development process.

docs/breaking_changes.rst

@@ -0,0 +1,45 @@
+.. Copyright 2013 – present by the SalishSeaCast Project Contributors
+.. and The University of British Columbia
+..
+.. Licensed under the Apache License, Version 2.0 (the "License");
+.. you may not use this file except in compliance with the License.
+.. You may obtain a copy of the License at
+..
+..    https://www.apache.org/licenses/LICENSE-2.0
+..
+.. Unless required by applicable law or agreed to in writing, software
+.. distributed under the License is distributed on an "AS IS" BASIS,
+.. WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+.. See the License for the specific language governing permissions and
+.. limitations under the License.
+
+.. SPDX-License-Identifier: Apache-2.0
+
+
+.. _toolsRepoChangesThatBreakBackwardCompatibility:
+
+*****************************************
+Changes That Break Backward Compatibility
+*****************************************
+
+.. _BreakingChangesVersion24.1:
+
+Version 24.1 (unreleased)
+=========================
+
+The following changes that were introduced in version 24.1 of the ``tools`` repository
+are incompatible with earlier versions:
+
+* Changed to `CalVer`_ versioning convention.
+  Version identifier format is now ``yy.n[.devn]``,
+  where ``yy`` is the (post-2000) year of release,
+  and ``n`` is the number of the release within the year, starting at ``1``.
+  After a release has been made the value of ``n`` is incremented by 1,
+  and ``.dev0`` is appended to the version identifier to indicate changes that will be
+  included in the next release.
+  ``24.1.dev0`` is an exception to that scheme.
+  That version identifies the period of development between the ``2.0`` and ``24.1``
+  releases.
+
+  .. _CalVer: https://calver.org/
+

docs/index.rst

@@ -38,6 +38,7 @@ Contents:
 .. toctree::
    :maxdepth: 2
 
+   breaking_changes
    SalishSeaTools/index
    SOGTools/index
    SalishSeaCmd/index