added summary

SUMMARY.md

@@ -0,0 +1,14 @@
+# Summary
+
+## Use headings to create page groups like this one
+
+* [First page's title](page1/README.md)
+    * [Some child page](page1/page1-1.md)
+    * [Some other child page](part1/page1-2.md)
+* [Second page's title](page2/README.md)
+    * [Some child page](page2/page2-1.md)
+    * [Some other child page](part2/page2-2.md)
+
+## A second page group
+
+* [Yet another page](another-page.md)