Add new concept topic, Patch Sets, to documentation

This concept topic provides conceptual information on what a patch set
is in Gerrit. Its intended audience is Gerrit users who, during the
course of learning how to use Gerrit, want to understand more about
what a Patch Set is, and some of the information the Gerrit Review
screen provides for a patch set.

This topic may eventually need screenshots, but creating them will happen in a
later change.

Change-Id: Icca726b598ce69593ff976c169bb4231244322a5
diff --git a/Documentation/index.txt b/Documentation/index.txt
index 70a6759..0f51926 100644
--- a/Documentation/index.txt
+++ b/Documentation/index.txt
@@ -94,6 +94,7 @@
 . link:access-control.html[Access Controls]
 . link:concept-changes.html[Changes]
 . link:concept-refs-for-namespace.html[The refs/for Namespace]
+. link:concept-patch-sets.html[Patch Sets]
 
 == Resources
 * link:licenses.html[Licenses and Notices]