dl: add LICENSE, AUTHORS, CONTRIBUTORS, PATENTS, CONTRIBUTING.md

These files should've been here when the repository was initially
created, but they were unintentionally forgotten. Better late than
never, so add them now.

Fixes golang/go#33942

Change-Id: I59fe6b9a3e2288089eb1211c5dc7f27b675f3d3a
Reviewed-on: https://go-review.googlesource.com/c/dl/+/195439
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
5 files changed