Add AUTO_OPTIN_REGEX_LIST_BASE64 to env doc

Author: engelhartrueben

docs/REFERENCE-environment_variables.md

@@ -10,6 +10,7 @@
 | AUTH0_DOMAIN                                     | Domain name on Auth0 account, should end in `.auth0.com`, e.g. `example.auth0.com`. _Required_.                                                                                                                                                                                                                                                                                             |
 | AUTH0_CLIENT_ID                                  | Client ID from Auth0 app. _Required_.                                                                                                                                                                                                                                                                                                                                                       |
 | AUTH0_CLIENT_SECRET                              | Client secret from Auth0 app. _Required_.                                                                                                                                                                                                                                                                                                                                                   |
+| AUTO_OPTIN_REGEX_LIST_BASE64 | JSON object encoded in base64 to specify opt in language following the structure: \`[{\"regex\": \"\",\"reason\": \"\"}]\`. |
 | AWS_ACCESS_AVAILABLE                             | 1 or 0 to enable or disable S3 campaign exports within Amazon Lambda.                                                                                                                                                                                                                                                                                                                       |
 | AWS_ACCESS_KEY_ID                                | AWS access key ID with access to S3 bucket, required for campaign exports outside Amazon Lambda.                                                                                                                                                                                                                                                                                            |
 | AWS_SECRET_ACCESS_KEY                            | AWS access key secret with access to S3 bucket, required for campaign exports outside Amazon Lambda.                                                                                                                                                                                                                                                                                        |