Add .gitignore

Add .gitignore file, so git commands ignore the following:

* .out/ (Build directory)
* .vscode/ (Visual Studio Code)
* .idea/ (Intellij)

Signed-off-by: Tim Van Patten <timvp@google.com>
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..f58f7a3
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+.out/
+.vscode/
+.idea/