docs/news: describe addition to the weldr API

The compose endpoint gained a new `url` parameter for building
ostree-based images.

Signed-off-by: Tom Gundersen <teg@jklm.no>

docs/news/unreleased/weldr-ostree-url.md

@@ -0,0 +1,40 @@
+# Weldr API: Allow parent OSTree commit to be read from repository
+
+The weldr API for building OSTree based images is extended to optionally take an `url` parameter instead of the current `parent`.
+
+The `parent` parameter contains the OSTree commit SHA of the parent commit when building an update commit. Obtaining this is cumbersome, so instead the `url` of the repository containing the desired parent commit can be specified. In this case, composer will take the current `HEAD` of the given `ref` as the parent.
+
+At most one of `parent` and `url` can be specified in a given compose request.
+
+Before:
+
+    curl --silent \
+        --header "Content-Type: application/json" \
+        --unix-socket /run/weldr/api.socket \
+        http://localhost/api/v1/compose \
+        --data "{ \
+            \"blueprint_name\": \"foo\", \
+            \"compose_type\": \"rhel-edge-commit\", \
+            \"ostree\": {\ \
+                \"parent\": \"b8a69e5c79be5830bb272356809a52b1660d2013c26f6973d549d0a312a8d21a\", \
+                \"ref\": \"fedora/stable/x86_64/iot\" \
+            } \
+        }"
+
+After:
+
+    curl --silent \
+        --header "Content-Type: application/json" \
+        --unix-socket /run/weldr/api.socket \
+        http://localhost/api/v1/compose \
+        --data "{ \
+            \"blueprint_name\": \"foo\", \
+            \"compose_type\": \"rhel-edge-commit\", \
+            \"ostree\": {\ \
+                \"url\": \"https://d2ju0wfl996cmc.cloudfront.net/\", \
+                \"ref\": \"fedora/stable/x86_64/iot\" \
+            } \
+        }"
+
+Relevant PRs:
+https://github.com/osbuild/osbuild-composer/pull/1235