go.mod: init Go module (#243)

diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..2e64352
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,10 @@
+module go.starlark.net
+
+go 1.13
+
+require (
+	github.com/chzyer/logex v1.1.10 // indirect
+	github.com/chzyer/readline v0.0.0-20180603132655-2972be24d48e
+	github.com/chzyer/test v0.0.0-20180213035817-a1ea475d72b1 // indirect
+	golang.org/x/sys v0.0.0-20191002063906-3421d5a6bb1c // indirect
+)