Clarify behavior of Disallow Indexing plugin

bedrock/deploying-bedrock.md

@@ -20,4 +20,6 @@ Any other deployment method can be used as well with one requirement:
 
 Although we recommend using [Trellis](https://roots.io/trellis/), it's not required in order to deploy a Bedrock site. 
 
-If you choose to use Capistrano for deployment, we offer a [screencast on deploying WordPress with Capistrano](https://roots.io/screencasts/deploying-wordpress-with-capistrano/) with more information.
+If you choose to use Capistrano for deployment, we offer a [screencast on deploying WordPress with Capistrano](https://roots.io/screencasts/deploying-wordpress-with-capistrano/) with more information.
+
+Whatever method you use to deploy, make sure that it involves setting `WP_ENV` to `production` when your site is in a production environment. Several things in the Roots stack, including the [Disallow Indexing mu-plugin](https://github.com/roots/bedrock/blob/master/web/app/mu-plugins/disallow-indexing.php) and [Soil](https://roots.io/plugins/soil/) rely on `WP_ENV` being set to the correct environment. The Disallow Indexing plugin, for instance, will prevent indexing of a site when `WP_ENV` is not set to `production`. For more details on `WP_ENV`, refer back to the "[Environment Variables](/bedrock/docs/environment-variables/)" section of this document.

bedrock/environment-variables.md

@@ -22,3 +22,13 @@ Currently, the following env vars are required:
 * `DB_PASSWORD`
 * `WP_HOME`
 * `WP_SITEURL`
+
+## WP_ENV
+
+Although it isn't required (if not defined elsewhere, Bedrock will default to `production`), `WP_ENV` is used by several pieces of the Roots stack, as well as software outside of it, to modify behavior based on environment. There are three values you can set for `WP_ENV` that Bedrock will understand:
+
+* `production`
+* `staging`
+* `development`
+
+Make sure that these are set correctly in your different environments.