Add missing parameter to ansible-playbook invocation

Add the necessary --extra-vars parameter to the instructions for
performing an installation.  The initial_run=true setting is
necessary to avoid a failure in the ElasticSearch setup.

README.md

@@ -80,7 +80,7 @@ $ ansible-playbook -i development -u vagrant \
 A second run to configure everything:
 ```
 $ ansible-playbook -i development -u <your username in group_vars/all> \
-  dev_all.yml
+  dev_all.yml --extra-vars "initial_run=true"
 ```
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