Adding back header section

Author: pwendell

docs/configuration.md

@@ -2,11 +2,17 @@
 layout: global
 title: Spark Configuration
 ---
-
 * This will become a table of contents (this text will be scraped).
 {:toc}
 
-Spark provides several locations to configure the system:
+Spark provides three locations to configure the system:
+
+* [Spark properties](#spark-properties) control most application parameters and can be set by passing
+  a [SparkConf](api/core/index.html#org.apache.spark.SparkConf) object to SparkContext, or through Java
+  system properties.
+* [Environment variables](#environment-variables) can be used to set per-machine settings, such as
+  the IP address, through the `conf/spark-env.sh` script on each node.
+* [Logging](#configuring-logging) can be configured through `log4j.properties`.
 
 # Spark Properties