Description for CustomPiwikJs plugin (#11380)

fixes #11291

plugins/CustomPiwikJs/lang/en.json

 {
     "CustomPiwikJs": {
+        "PluginDescription": "Allows any plugin to extend the Piwik JavaScript Tracking file (piwik.js) and add new functionnality and website measurement capabilities.",
         "DiagnosticPiwikJsWritable": "Writable JavaScript Tracker (\"/piwik.js\")",
         "DiagnosticPiwikJsNotWritable": "The Piwik JavaScript tracker file \"/piwik.js\" is not writable which means other plugins cannot extend the JavaScript tracker. In the future even some core features might not work as expected. ",
         "DiagnosticPiwikJsMakeWritable": "We recommend to Piwik.js writable by running this command: %s"