Add noboolean example

Patch by brandonwilliams; reviewed by bereng for CASSANDRA-18059

site-content/source/modules/ROOT/pages/download.adoc

@@ -168,6 +168,7 @@ and repeat `sudo apt-get update`. The actual key may be different, you get it fr
 * The latest `<release series>` is `40x`.
 * For older releases, the `<release series>` can be one of `311x`, `30x`, or `22x`.
 * (Not all versions of Apache Cassandra are available, since building RPMs is a recent addition to the project.)
+* For CentOS 7 and similar (rpm < 4.14), append the `noboolean` repository
 * Add the Apache repository of Cassandra to `/etc/yum.repos.d/cassandra.repo`, for example for the latest 4.0 version:
 
 
@@ -179,8 +180,20 @@ baseurl=https://redhat.cassandra.apache.org/40x/
 gpgcheck=1
 repo_gpgcheck=1
 gpgkey=https://downloads.apache.org/cassandra/KEYS
+--
+
+Or for CentOS 7:
 
+[source]
+--
+[cassandra]
+name=Apache Cassandra
+baseurl=https://redhat.cassandra.apache.org/40x/noboolean/
+gpgcheck=1
+repo_gpgcheck=1
+gpgkey=https://downloads.apache.org/cassandra/KEYS
 --
+
 * Install Cassandra, accepting the gpg key import prompts:
 
 [source]