all: add go.mod

Updates golang/go#30228

Change-Id: Idde6e34da043b401ee7429a0297517ec235e3b64
Reviewed-on: https://go-review.googlesource.com/c/arch/+/167137
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..7edc03e
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,5 @@
+module golang.org/x/arch
+
+go 1.11
+
+require rsc.io/pdf v0.1.1
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..e854d25
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,2 @@
+rsc.io/pdf v0.1.1 h1:k1MczvYDUvJBe93bYd7wrZLLUEcLZAuF824/I4e5Xr4=
+rsc.io/pdf v0.1.1/go.mod h1:n8OzWcQ6Sp37PL01nO98y4iUCRdTGarVfzxY20ICaU4=