Add description of Github Actions (#138)

* Create Process_CI.md

docs/Process_CI.md

@@ -0,0 +1,57 @@
+# Build pipelines
+
+<br>
+
+Armbian Build pipelines are combination of Github Actions and scripts that run on our servers.  Armbian is providing a large quantity of compiled binaries and images which builds are distributed over our self-hosted build farm.
+
+<br>
+
+## Nightly builds 
+
+<br>
+
+![Build](images/main-pipeline.png)
+
+- edge branch in https://apt.armbian.com
+- all branches in https://beta.armbian.com
+- rootfs cache
+<br>
+ The repository indexes for BETA are updated immediately.
+
+<br>
+<br>
+Trigger: every day at 6am CET
+<br>
+Condition: change in packages, upstream sources, patches or configuration
+
+## Updating all beta images
+
+<br>
+
+![Updating all beta images](images/betaimages.png)
+
+<br>
+
+- triggered manually or uppon completion of nightly / edge builds;
+- running the job manual is possible,
+- pipeline is always using packages from https://beta.armbian.com repository.
+
+<br>
+
+## Updating selected stable images
+<br>
+<br>
+
+If you have a commit rights to the repository, go to [Armbian build system actions](https://github.com/armbian/build/actions) and select *Build selected*:
+
+<br>
+
+![Updating selected stable images](images/build-selected-blured.png)
+
+You can recreate image(s) from sources - set `packages from repository` to *no* - or from packages that are already in repository (default). In case you choose to build from sources, stable https://apt.armbian.com repository is going to be populated with newly created u-boot, kernel and **BSP packages for all boards** under (patched) stable version (yy.mm.**x+1**) which is incremented automatically if process succeeds.
+
+![kanban screenshot](images/stable-images.png)
+
+<br>
+
+When new artifacts are created for stable builds, content is uploaded to CDN, then download and repository indexes are updated.  The process is typically complete in 1 to 2 days for major releases.

mkdocs.yml

@@ -79,6 +79,7 @@ nav:
   - "Contributor Process" :
         - "Contribute" : "Process_Contribute.md"
         - "Merge Policy" : "Process_Merge-Policy.md"
+        - "Continuous integration" : "Process_CI.md"
         - "Documentation" : "Process_Documentation.md"
 
   - "Release management" :