Add chapter about how to pass in arbitrary variables. This closes #478

478-backup-arbitrary-variables.patch

diff --git a/Performing-Backups.md b/Performing-Backups.md
index d5e1511..472e9a2 100644
--- a/Performing-Backups.md
+++ b/Performing-Backups.md
@@ -191,6 +191,24 @@ The `backup perform` command will exit with the following status codes:
 **2**: All triggers were _processed_, but some failed.
 **3**: A fatal error caused Backup to exit. Some triggers may not have been processed.
 
+Passing Arbitrary Variables
+---------------------------
+
+If you wish to pass in parameters other then the predefined Command Line
+Options, you can do so using environment-variables. For example:
+
+    $ DB_NAME=my_app_production backup perform --trigger my_backup
+
+Then, you can access these in the `my_backup.rb` model:
+
+    Backup::Model.new(:my_backup, 'Description for my_backup') do
+      database MongoDB do |db|
+        db.name = ENV["DB_NAME"] || "default_name"
+        #...
+      end
+    end
+
+This allows for a more dynamic backup-plan.
 
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