doc(release): document 6.x manual release process

Signed-off-by: Marc Nuri <[email protected]>

Author: manusa

doc/RELEASE.md

@@ -0,0 +1,41 @@
+## Kubernetes Client 6.x
+
+The CI pipeline for the Kubernetes Client 6.x is outdated and no longer functional with the new Sonatype Maven Central.
+The main issue is that since the release process requires compiling the project with multiple JDK versions in split processes,
+the new `central-publishing-maven-plugin` does not support this workflow.
+
+To be able to still release the Kubernetes Client 6.x, we have to use a manual process.
+
+> [!NOTE]
+> The SNAPSHOTs pipeline is still working since the SNAPSHOT release process is similar to that of the OSSRH repository.
+
+### Manual Release Process
+
+#### Build and install the project
+
+Run the following command to build and install the project locally:
+
+```bash
+PASSPHRASE=<your_passphrase> ./scripts/6.x-release.sh
+```
+
+`PASSPHRASE` is the passphrase for the GPG key used to sign the artifacts.
+
+#### Generate the ZIP bundle
+
+Run the following command to generate the ZIP bundle:
+
+```bash
+./scripts/6.x-bundle.sh
+```
+
+This will create a ZIP file in a `/tmp/tmp.xxxxxxxxxxx` directory with the name `kubernetes-client-6.x.x-bundle.zip`.
+
+#### Upload the ZIP bundle to Sonatype
+
+Upload the generated ZIP bundle to Sonatype Maven Central using their web interface.
+
+https://central.sonatype.com/publishing/deployments
+
+Ensure that it validates and then click on "Publish".
+