Add combinatorics summary until topic 3

Change-Id: Iad09497c56db0301b7b4f25a1de4192ab1b1929c
diff --git a/quad9/combinatorics/summary/.gitignore b/quad9/combinatorics/summary/.gitignore
new file mode 100644
index 0000000..120e611
--- /dev/null
+++ b/quad9/combinatorics/summary/.gitignore
@@ -0,0 +1,2 @@
+*.pdf
+figures/