Document security privileges required for using Watcher (elastic/x-pack-elasticsearch#1012)

* Document security privileges required for using Watcher

* Fixing index names

* Language fixes

Original commit: elastic/x-pack-elasticsearch@56ef06d

Author: ycombinator

docs/en/watcher/getting-started.asciidoc

@@ -211,6 +211,14 @@ DELETE _xpack/watcher/watch/log_error_watch
 // CONSOLE
 // TEST[continued]
 
+[float]
+[[required-security-privileges]]
+=== Required Security Privileges
+To use Watcher, users must have the following security privileges:
+
+* Cluster `manage` privilege. Enables users to access the Watcher APIs.
+* Index `read` privilege on `.watch*` indices. Enables users to read the `.watches` and .`watcher-history-*` indices.
+
 [float]
 [[next-steps]]
 === Where to Go Next