Add .gitignore to skip generated HTML files under _site/*

Change-Id: I3352aefb0a3a87af485aae5c06bcf29c4d791887

.gitignore

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..1643cc2
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+.DS_Store
+_site/*