all: add go.mod file

Change-Id: Ia89984a50f6147ef71acb7a733c25f8d6f2dbd74
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..e180d70
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,10 @@
+module cuelang.org/go
+
+require (
+	github.com/cockroachdb/apd v1.1.0
+	github.com/ghodss/yaml v1.0.0
+	github.com/pkg/errors v0.8.0 // indirect
+	golang.org/x/exp/errors v0.0.0-20181210123644-7d6377eee41f
+	golang.org/x/tools v0.0.0-20181210225255-6a3e9aa2ab77
+	gopkg.in/yaml.v2 v2.2.2 // indirect
+)