some docs updates

docs/_docs/02-03-plugins.md

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+---
+title: Plugins
+permalink: /docs/running/plugins/
+---
+
+Configuration for the available plugins are stored in a JSON encoded string. The format
+of this document are available with the [Plugins Overview]({{ "/docs/plugins/" | absolute_url }}).
+
+You can override the plugin config by specifying the content in the `TALK_PLUGIN_JSON`
+environment variable.
+
+## Bundled Plugin Configuration
+{:.no_toc}
+
+Some of the core plugins that are bundled with Talk require specific configuration to be
+available.
+
+{% include toc %} 
+
+### Facebook Authentication
+
+- `TALK_FACEBOOK_APP_ID` (*required*) - the Facebook app id for your Facebook
+Login enabled app.
+- `TALK_FACEBOOK_APP_SECRET` (*required*) - the Facebook app secret for your
+Facebook Login enabled app.