OOB_5.12 Clarification regarding label directories (#1021)

GuyWi · web-flow · commit f22f8b35f9cc · 2019-03-20T19:42:45.000+02:00
* Update build_rules.md

* Update based on Jimmy's input.

* Update build_rules.md

* Update build_rules.md

* Update build_rules.md
diff --git a/docs/reference/build_rules.md b/docs/reference/build_rules.md
@@ -30,6 +30,10 @@ Label directories are directories that follow a naming convention: an upper case
 - `COMPONENT`, controlled from the configuration value `target.components`.
 - `TOOLCHAIN`, controlled completely by the toolchain used to build.
 
+The Mbed OS build tools skip all label directories, unless you configure them explicitly, as described above.
+
+For more information about using, extending, and removing labels, see [Adding and configuring targets](../reference/adding-and-configuring-targets.html#extra-labels-extra-labels-add-and-extra-labels-remove).
+
 #### Target directories
 
 The target labeled directories are used for selecting sources specific to an MCU or MCU family. The set of directories included in a given build is all of the names of the targets in the inheritance hierarchy and the `target.extra_labels`. The following is an example `targets.json`:
