Add api deprecation policy

docs/api.md

@@ -541,6 +541,12 @@ server, the following procedure should be performed:
 2.  Determine whether the client is compatible with this server based on
     the presence/absence of these headers and their values if present.
 
+### API Version Deprecation Policy
+
+Older API versions are guaranteed to be supported for at least one year
+after the release of a new API version. After that, support for older
+API versions may be (but is not guaranteed to be) dropped.
+
 ### API Changelog
 
 #### Version 1

src/paperless/settings.py

@@ -343,7 +343,7 @@ REST_FRAMEWORK = {
     "DEFAULT_VERSIONING_CLASS": "rest_framework.versioning.AcceptHeaderVersioning",
     "DEFAULT_VERSION": "7",
     # Make sure these are ordered and that the most recent version appears
-    # last
+    # last. See api.md#api-versioning when adding new versions.
     "ALLOWED_VERSIONS": ["1", "2", "3", "4", "5", "6", "7"],
 }