Add RELEASING.md

RELEASING.md

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+# Releasing
+
+Every push to `master` runs `maven_release.yml`, which builds, signs with the
+organisation key and publishes to Maven Central. While the version in `pom.xml`
+ends in `-SNAPSHOT` that publishes a snapshot; a release is the same push with a
+release version. So a release is three pull requests and one tag, the way 0.2.11
+was cut in #168 and 0.2.12 in #194.
+
+## Before
+
+- `master` is green on JDK 17, 21 and 25, and the last `maven_release.yml` run
+  on `master` succeeded.
+- Every change meant for the release is merged. Nothing merges on its author's
+  approval alone: one approval from another collaborator, or no objection after
+  a week (issue #71).
+- The release notes are drafted from the merged pull requests, with authors
+  credited by handle.
+
+## Steps
+
+1. Open a pull request titled `0.2.N release` that changes the version in
+   `pom.xml` from `0.2.N-SNAPSHOT` to `0.2.N` and the scm `<tag>` from
+   `jackson-databind-nullable-0.2.N-SNAPSHOT` to
+   `jackson-databind-nullable-0.2.N`. Get it approved like any other change,
+   then merge it. The push to `master` publishes `0.2.N`
+   to Maven Central; wait for the workflow to finish and for the version to
+   appear at
+   https://repo1.maven.org/maven2/org/openapitools/jackson-databind-nullable/.
+2. Tag the merge commit `v0.2.N` and push the tag. Create the GitHub release
+   from the tag with the drafted notes.
+3. Open and merge a pull request `prepare 0.2.N+1 snapshot` that sets the
+   version to `0.2.N+1-SNAPSHOT` and the scm `<tag>` to
+   `jackson-databind-nullable-0.2.N+1-SNAPSHOT`. Nothing else merges between
+   steps 1 and 3, Dependabot bumps included: every push to `master` runs the
+   deploy again, and a second publish of `0.2.N` fails.
+4. Open the version bump in OpenAPITools/openapi-generator, where the Java
+   templates pin `jackson-databind-nullable-version`, as #24501 did for 0.2.11.
+
+## If something goes wrong
+
+If the build job of `maven_release.yml` fails on the release commit, nothing is
+published; fix `master` and push again, the workflow reruns on every push. If
+the publish step itself failed, check Maven Central and the Central Portal
+before pushing again: the upload may still go through. A version that reached
+Maven Central cannot be removed; ship `0.2.N+1` with the fix instead.