playground: prevent accidentally commiting playground netrc

Change-Id: I34cc398f995def2f038c9ec4c0ba23f8a2cdfff2
diff --git a/tools/playground/.gitignore b/tools/playground/.gitignore
new file mode 100644
index 0000000..a25285a
--- /dev/null
+++ b/tools/playground/.gitignore
@@ -0,0 +1 @@
+builder/netrc