add release mixin doc

doc/ReleaseMixin.md

@@ -0,0 +1,81 @@
+# ReleaseMixin
+
+Support for Releases following the trunk-based-release flow (see https://trunkbaseddevelopment.com/)
+
+```mermaid
+classDiagram
+    class ReleaseMixin {
+        prepare_release() - adjust all build files to carry the correct version & commit localy
+        tag_and_push_release() - tag the git repo and push changes to origin
+    }
+
+```
+
+## Input
+
+| name                          | description                                                                                                           | default         |
+| ----------------------------- | --------------------------------------------------------------------------------------------------------------------- | --------------- |
+| release_type                  | one of MAJOR, MINOR, PATCH, NONE                                                                                      | "NONE"          |
+| release_main_branch           | the name of your trank                                                                                                | "main"          |
+| release_primary_build_file    | path to the build file having the leading version info (read & write). Valid extensions are .clj, .json, .gradle, .py | "./project.clj" |
+| release_secondary_build_files | list of secondary build files, version is written in.                                                                 | []              |
+
+## Example Usage
+
+### build.py
+
+```python
+rom os import environ
+from pybuilder.core import task, init
+from ddadevops import *
+
+name = 'my-project'
+MODULE = 'my-module'
+PROJECT_ROOT_PATH = '..'
+
+@init
+def initialize(project):
+    project.build_depends_on("ddadevops>=4.0.0")
+
+    input = {
+        "name": name,
+        "module": MODULE,
+        "stage": "notused",
+        "project_root_path": PROJECT_ROOT_PATH,
+        "build_types": [],
+        "mixin_types": ["RELEASE"],
+        "release_type": "MINOR",
+        "release_primary_build_file": "project.clj",
+        "release_secondary_build_files": ["package.json"],
+    }
+
+    roject.build_depends_on("ddadevops>=4.0.0-dev")
+    
+    build = ReleaseMixin(project, input)
+    build.initialize_build_dir()
+
+
+@task
+def prepare_release(project):
+    build = get_devops_build(project)
+    build.prepare_release()
+
+@task
+def build(project):
+    print("do the build")
+
+@task
+def publish(project):
+    print("publish your artefacts")
+
+@task
+def after_publish(project):
+    build = get_devops_build(project)
+    build.tag_bump_and_push_release()
+```
+
+### call the build
+
+```bash
+pyb prepare_release build publish after_publish
+```