Add a release note about import locking

This adds a release note detailing the new locking behavior and criteria
for stealing the lock.

Related-Bug: #1884596
Change-Id: I19c713c91794694f990f1372fda61cc2e20fac54

releasenotes/notes/import-locking-behavior-901c691f3839fe0a.yaml

@@ -0,0 +1,14 @@
+---
+fixes:
+  - |
+    A change was added to the import API which provides time-based
+    locking of an image to exclude other import operations from
+    starting until the lock-holding task completes (see Bug
+    1884596_). The lock is based on the task that we start to do the
+    work, and the UUID of that task is stored in the
+    ``os_glance_import_task`` image property, which indicates who owns
+    the lock. If the task holding the lock fails to make progress for
+    60 minutes, another import operation will be allowed to steal the
+    lock and start another import operation.
+
+    .. _1884596: https://bugs.launchpad.net/glance/+bug/1884596