go: Create a go.mod at the top

This module file tells the Go tools the URL space where this project
lives.  This allows code within the project to refer to these files by
URL.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..bdb354f
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module github.com/JuulLabs-OSS/mcuboot
+
+go 1.14