Add a brief description of purpose

This event is both about improving Git the community and Git the
tool.

Change-Id: Ic58c31ebd98c25ea5ead983ff996c625bde4729b
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

index.md

diff --git a/index.md b/index.md
index a00cb2c..fe4e786 100644
--- a/index.md
+++ b/index.md
@@ -3,7 +3,16 @@
 Monday, September 14th 9am-12:30 pm PDT \
 Tuesday, September 15th 10am-2pm PDT
 
-## About
+## Purpose
+
+This summit is part of a broader effort to make the Git version
+control system support inclusive open source projects and their
+contributors.
+
+Git needs to take the needs of all its users into account, not just
+those currently participating in the Git project.
+
+## Overview
 
 Git Inclusion Summit is a partnership between the Git Core team at
 Google, Google OSPO (Open Source Program Office), and Microsoft OSPO.