BUILD-4860 Document the release process of parent-oss project (#159)

That way, collaborators have a guide and we can potentially automate it later.

README.md

@@ -8,3 +8,5 @@ Licensed under the [GNU Lesser General Public License, Version 3.0](http://www.g
 
 After the build artifacts get promoted to the releases repository on repox, 
 they get automatically uploaded to Maven Central.
+
+The release process is described in [RELEASE.md](./RELEASE.md)

RELEASE.md

@@ -0,0 +1,20 @@
+# Releasing the parent-oss project
+
+> WARN: Due to some internal tooling (releasability checks) semantic versioning is barely supported.
+> 
+> Therefore, a new release number has to be a new **major**.
+
+Assume you want to release from version `70.0.0.x`, 
+**the next version must be** `71.0.0.x`
+1. Prepare a new project release in [Jira](https://sonarsource.atlassian.net/projects/PARENTOSS?selectedItem=com.atlassian.jira.jira-projects-plugin%3Arelease-page) with as version `71.0` (no patch or build number)
+
+2. Leave the Jira version status as `UNRELEASED`
+3. Update pom.xml version of parent-oss project. (example [PR](https://github.com/SonarSource/parent-oss/pull/158/files))
+4. Check that releasability checks pass on [Burgr](https://burgr.sonarsource.com/projects/SonarSource/parent-oss/main)
+5. Retrieve the last build number on [Burgr](https://burgr.sonarsource.com/projects/SonarSource/parent-oss/main) (`major.minor.patch.build-number`)
+6. On GitHub create a new release and set this number retrieved from Burgr as tag and release version
+7. Publish the release
+8. Check that the [GitHub release workflow](https://github.com/SonarSource/parent-oss/actions/workflows/release.yml) run well 
+9. Check it is gracefully deployed on [Sonatype](https://central.sonatype.com/artifact/org.sonarsource.parent/parent). 
+
+> WARN: It can take up to 24h to have the release synchronized with Sonatype. Sometimes it is very fast sometimes not)