Fix two download suggestions in the docs:

1) On the quick start page provide a direct link to the downloads (suggested by @pbailis).
2) On the index page, don't suggest users always have to build Spark, since many won't.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #662 from pwendell/quick-start and squashes the following commits:

0622f27 [Patrick Wendell] Fix two download suggestions in the docs:

1 files changed, 10 insertions, 26 deletions

diff --git a/docs/index.md b/docs/index.md
index 2daa208b3b..e3647717a1 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -9,17 +9,18 @@ It also supports a rich set of higher-level tools including [Shark](http://shark
 
 # Downloading
 
-Get Spark by visiting the [downloads page](http://spark.apache.org/downloads.html) of the Apache Spark site. This documentation is for Spark version {{site.SPARK_VERSION}}.
+Get Spark by visiting the [downloads page](http://spark.apache.org/downloads.html) of the Apache Spark site. This documentation is for Spark version {{site.SPARK_VERSION}}. The downloads page 
+contains Spark packages for many popular HDFS versions. If you'd like to build Spark from 
+scratch, visit the [building with Maven](building-with-maven.html) page.
 
-Spark runs on both Windows and UNIX-like systems (e.g. Linux, Mac OS). All you need to run it is to have `java` to installed on your system `PATH`, or the `JAVA_HOME` environment variable pointing to a Java installation.
+Spark runs on both Windows and UNIX-like systems (e.g. Linux, Mac OS). All you need to run it is 
+to have `java` to installed on your system `PATH`, or the `JAVA_HOME` environment variable 
+pointing to a Java installation.
 
-# Building
-
-Spark uses [Simple Build Tool](http://www.scala-sbt.org), which is bundled with it. To compile the code, go into the top-level Spark directory and run
-
-    sbt/sbt assembly
-
-For its Scala API, Spark {{site.SPARK_VERSION}} depends on Scala {{site.SCALA_BINARY_VERSION}}. If you write applications in Scala, you will need to use a compatible Scala version (e.g. {{site.SCALA_BINARY_VERSION}}.X) -- newer major versions may not work. You can get the right version of Scala from [scala-lang.org](http://www.scala-lang.org/download/).
+For its Scala API, Spark {{site.SPARK_VERSION}} depends on Scala {{site.SCALA_BINARY_VERSION}}. 
+If you write applications in Scala, you will need to use a compatible Scala version 
+(e.g. {{site.SCALA_BINARY_VERSION}}.X) -- newer major versions may not work. You can get the 
+right version of Scala from [scala-lang.org](http://www.scala-lang.org/download/).
 
 # Running the Examples and Shell
 
@@ -50,23 +51,6 @@ options for deployment:
 * [Apache Mesos](running-on-mesos.html)
 * [Hadoop YARN](running-on-yarn.html)
 
-# A Note About Hadoop Versions
-
-Spark uses the Hadoop-client library to talk to HDFS and other Hadoop-supported
-storage systems. Because the HDFS protocol has changed in different versions of
-Hadoop, you must build Spark against the same version that your cluster uses.
-By default, Spark links to Hadoop 1.0.4. You can change this by setting the
-`SPARK_HADOOP_VERSION` variable when compiling:
-
-    SPARK_HADOOP_VERSION=2.2.0 sbt/sbt assembly
-
-In addition, if you wish to run Spark on [YARN](running-on-yarn.html), set
-`SPARK_YARN` to `true`:
-
-    SPARK_HADOOP_VERSION=2.0.5-alpha SPARK_YARN=true sbt/sbt assembly
-
-Note that on Windows, you need to set the environment variables on separate lines, e.g., `set SPARK_HADOOP_VERSION=1.2.1`.
-
 # Where to Go from Here
 
 **Programming guides:**