all: add licensing boilerplate and update README

Change-Id: I79bcdc1d868fccbb778ccdef23a4ad7389cf5bfe
Reviewed-on: https://team-review.git.corp.google.com/c/golang/vulndb/+/1054183
Reviewed-by: Filippo Valsorda <valsorda@google.com>
diff --git a/CONTRIBUTORS b/CONTRIBUTORS
new file mode 100644
index 0000000..1fbd3e9
--- /dev/null
+++ b/CONTRIBUTORS
@@ -0,0 +1,3 @@
+# This source code was written by the Go contributors.
+# The master list of contributors is in the main Go distribution,
+# visible at https://tip.golang.org/CONTRIBUTORS.