Add pipelines doc

Add explanation of the explicit CI triggers in the pipelines.

Signed-off-by: Petr Kotas <pkotas@redhat.com>

docs/pipelines.md

@@ -0,0 +1,16 @@
+# Pipelines
+
+CI is running on the Azure Dev Ops Pipelines, which is configured in the
+`.pipelines` directory.
+
+Documentation for the pipelines is located in the [MSDN](https://docs.microsoft.com/en-us/azure/devops/pipelines/?view=azure-devops).
+
+
+## Pipelines triggers
+
+**e2e** are configured to be explicitly run when new commit is done on `master` branch, excluding changes for files in `docs/*`.
+Default pipelines configuration is implicitly doing this, but for all folders.
+
+For PRs the **e2e** is triggered also when it should be merged to the `master`, but excludes changes in `docs/*`.
+
+This saves testing infrastructure cycles for simple documentation updates.