Add AUTHORS file.

AUTHORS

@@ -0,0 +1,17 @@
+# This is the official list of Tailscale authors for copyright purposes.
+#
+# Names should be added to this file as one of
+#
+#     Organization's name
+#     Individual's name <submission email address>
+#     Individual's name <submission email address> <email2> <emailN>
+#
+# Please keep the list sorted.
+#
+# You do not need to add entries to this list, and we don't actively populate
+# this list. If you do want to be acknowledged explicitly as a copyright
+# holder, though, then please send a PR referencing your earlier contributions
+# and clarifying whether it's you or your company that owns the rights to your
+# contribution.
+
+Tailscale Inc.