Add a doc on how to include an upstream Theia patch into the Che-Theia release

RELEASE.md

@@ -1,3 +1,30 @@
-# Eclipse Che-Theia release process
+## Eclipse Che-Theia release process
+See [Release Che-Theia](./.github/workflows/release.yml) workflow.
 
-See [Release Che Theia](./.github/workflows/release.yml) workflow.
+## Bugfix release with the upstream Theia commit included
+To make another bugfix release of Che-Theia with the upstream Theia patch included, there're several options available.
+
+### To include all the commits made before the required patch
+- switch to the required Che-Theia tag, e.g. [7.27.x](https://github.com/eclipse/che-theia/blob/7.27.x)
+- update the `build.include` file to match upstream Eclipse Theia revision:
+  - `THEIA_COMMIT_SHA="<commit-sha>"`
+- continue the release process as usual
+
+### To include only the required upstream commit
+- clone the Theia fork https://github.com/redhat-developer/eclipse-theia
+- sync the `master` branch of the forked repository with the origin one and push the changes
+- take the Theia commit sha which was used for a Che-Theia release, e.g. for 7.27.x it's [`1110f990`](https://github.com/eclipse/che-theia/blob/7.27.x/build.include#L17)
+- create a new branch
+- cherry-pick the required commit sha to the created branch and push the changes
+- update the `build.include` file:
+  - `THEIA_GITHUB_REPO="redhat-developer/codeready-workspaces-theia"`
+  - `THEIA_BRANCH="<created-branch-name>"`
+  - `THEIA_COMMIT_SHA="<commit-sha>"`
+- push the changes
+- continue the release process as usual
+
+### Include a diff file
+It's also possible to use a diff file to patch the Theia while releasing Che-Theia:
+- prepare a `*.patch` file
+- put it into the [`patches`](https://github.com/eclipse/che-theia/tree/master/dockerfiles/theia/src/patches) folder and a sub-folder which name corresponds to the [`THEIA_VERSION`](https://github.com/eclipse/che-theia/blob/7.27.x/build.include#L15) value, e.g. `master`
+- continue the release process as usual