[luci-go] Add go.mod

Required to use replace statement in fuchsia/tools/go.mod.

Bug: 58804
Change-Id: Ide26d56198a586c0f42fec8fdb1635bcc8a9a7b7
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/luci-go/+/419974
Reviewed-by: Nicolas Sylvain <nsylvain@google.com>
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..c8f5944
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module go.chromium.org/luci
+
+go 1.15