docs: add breaking changes documentation

Signed-off-by: simonsan <[email protected]>

src/SUMMARY.md

@@ -4,6 +4,7 @@
 - [Contributing to rustic](./contributing-to-rustic.md)
 - [FAQ](./FAQ.md)
   - [Comparison `rustic` and `restic`](./comparison-restic.md)
+- [Breaking Changes](./breaking_changes.md)
 - [Installation](./installation.md)
   - [Nightly Builds](./nightly_builds.md)
 - [Completions](./commands/misc/completions.md)

src/breaking_changes.md

@@ -0,0 +1,22 @@
+# Breaking Changes
+
+This document lists all user facing breaking changes in `rustic` and provides
+guidance on how to migrate from one version to another.
+
+## 0.9.0
+
+### Configuration File
+
+Consistently apply singular and plural naming conventions for keys in the
+configuration file that accept an array of values. This change affects the
+following keys:
+
+- glob -> globs
+- iglob -> iglobs
+- glob_file -> glob_files
+- iglob_file -> iglob_files
+- custom_ignorefile -> custom_ignorefiles
+- tag -> tags
+
+You need to update your configuration file accordingly, changing the key names
+from singular to plural.