Document custom plugin environment variables.

Author: mattk42

README.md

@@ -46,3 +46,16 @@ Deis output is a custom fluentd plugin that was written to forward data directly
 The topics these messages are put on are configurable via environment variables.
 * `NSQ_LOG_TOPIC`
 * `NSQ_METRIC_TOPIC`
+
+### Custom Plugins
+If you need something beyond the plugins that come pre-installed in the image, it is possible to set some environment variables to install and configure custom plugins as well.
+
+To install a custom plugin, simply set a FLUENTD_PLUGIN_# environment variable. For multiple plugins simply increment the trailing number.
+`FLUENTD_PLUGIN_1=some-fluentd-plugin`
+
+To configure your custom plugins, use either the CUSTOM_STORE_# or CUSTOM_FILTER_# environment variables
+* `CUSTOM_STORE_1="configuration text"`
+* `CUSTOM_FILTER_1="configuration text"`
+
+If you need the build tools available for installing your plugin, this can be enabled with another environment variable
+`INSTALL_BUILD_TOOLS="true"`