Adds documentation for BUNDLE_APP_CONFIG env var

Fixes rubygems#7085

man/bundle-config.ronn

@@ -11,7 +11,7 @@ This command allows you to interact with Bundler's configuration system.
 
 Bundler loads configuration settings in this order:
 
-1. Local config (`app/.bundle/config`)
+1. Local config (`app/.bundle/config` or `ENV['BUNDLE_APP_CONFIG']/.bundle/config`)
 2. Environmental variables (`ENV`)
 3. Global config (`~/.bundle/config`)
 4. Bundler default config
@@ -31,7 +31,9 @@ overridden and user will be warned.
 Executing `bundle config set --global <name> <value>` works the same as above.
 
 Executing `bundle config set --local <name> <value>` will set that configuration to
-the local application. The configuration will be stored in `app/.bundle/config`.
+the local application. The configuration will be stored in `app/.bundle/config`. If
+`BUNDLE_APP_CONFIG` is set, the configuration will be stored in 
+`$BUNDLE_APP_CONFIG/.bundle/config`.
 
 Executing `bundle config unset <name>` will delete the configuration in both
 local and global sources.