Add CI API prefix to documentation

[ci skip]

doc/ci/api/README.md

@@ -8,6 +8,14 @@ GitLab CI Runners.
 For consumer API take a look at this [documentation](../../api/README.md) where
 you will find all relevant information.
+## API Prefix
+
+Current CI API prefix is `/ci/api/v1`.
+
+You need to prepend this prefix to all examples in this documentation, like:
+
+    GET /ci/api/v1/builds/:id/artifacts
+
 ## Resources
 - [Builds](builds.md)