[sysprof] docs: update DESIGN.md

[Christian Hergert <chergert src gnome org>]

commit 82d924b87aca09968681ce4c0b9dd6e7b76496f0
Author: Christian Hergert <chergert redhat com>
Date:   Fri Apr 22 18:34:48 2016 -0700

    docs: update DESIGN.md

 DESIGN.md |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)
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diff --git a/DESIGN.md b/DESIGN.md
index 44e2dde..c5a6911 100644
--- a/DESIGN.md
+++ b/DESIGN.md
@@ -4,6 +4,15 @@
 
 ### libsysprof-2
 
+This library provides various profiling infrastructure for Sysprof.
+This includes the core interfaces for data sources, capture readers
+and writers, and the SpProfiler implementation. It also includes
+the SpProfile interface and its various implementations, such as
+SpCallgraphProfile. If you need a no-frills library to perform
+captures this is all you need.
+
+### libsysprof-ui-2
+
 This library provides various UI components and profiling tools as a library.
 It is intended to be used from IDEs such as GNOME Builder.