Add the option to wait for another service/s before starting (systemd only)

PROD-34894

Change-Id: Ifac85c9180fd3eb87c52460342988581bf244d09
diff --git a/README.rst b/README.rst
index 2f60808..c12d847 100644
--- a/README.rst
+++ b/README.rst
@@ -531,6 +531,18 @@
                order: 'allow,deny'
                allow: 'from all'
 
+Apache2 configured to wait for another service/s before
+starting (currently only with systemd):
+
+.. code-block:: yaml
+
+    apache:
+      server:
+        wait_for_service:
+          - foo-bar.mount
+        enabled: true
+        ...
+
 More Information
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