Add minimal go.mod

Since it's a fork, there isn't much else to do. This will enable the use of a
replace statement in fuchsia.git's go.mod files.

Bug: 58804
Change-Id: I055c49f4a6b6ed181bb8774abfc9f28f4375c894
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/go-tuf/+/420456
Reviewed-by: Erick Tryzelaar <etryzelaar@google.com>
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..f3a3642
--- /dev/null
+++ b/go.mod
@@ -0,0 +1 @@
+module github.com/flynn/go-tuf