[BUILD] Adds URLIndicator along with documentation

Adds URLIndicator to the build, testable adding

`openmct.install(new openmct.plugins.URLIndicatorPlugin({
url: 'http://localhost:8080/',
icon: 'check',
interval: 15000,
label: 'Localhost'
}))`

to the openmct file. Also added Documentation about the plugin.

src/plugins/plugins.js

@@ -27,7 +27,8 @@ define([
     '../../platform/features/autoflow/plugin',
     './timeConductor/plugin',
     '../../example/imagery/plugin',
-    '../../platform/import-export/bundle'
+    '../../platform/import-export/bundle',
+    './URLIndicatorPlugin/URLIndicatorPlugin'
 ], function (
     _,
     UTCTimeSystem,
@@ -35,7 +36,8 @@ define([
     AutoflowPlugin,
     TimeConductorPlugin,
     ExampleImagery,
-    ImportExport
+    ImportExport,
+    URLIndicatorPlugin
 ) {
     var bundleMap = {
         CouchDB: 'platform/persistence/couch',
@@ -121,5 +123,7 @@ define([
 
     plugins.ExampleImagery = ExampleImagery;
 
+    plugins.URLIndicatorPlugin = URLIndicatorPlugin;
+
     return plugins;
 });