doc(field_theory/normal): Add authors (#6759)

Adds Patrick Lutz and I as authors to normal.lean. The last three-quarters of the file are from our work on Abel-Ruffini.

https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/Author.20on.20normal.2Elean.3F

src/field_theory/normal.lean

@@ -1,7 +1,7 @@
 /-
 Copyright (c) 2020 Kenny Lau. All rights reserved.
 Released under Apache 2.0 license as described in the file LICENSE.
-Authors: Kenny Lau
+Authors: Kenny Lau, Thomas Browning, Patrick Lutz
 -/
 
 import field_theory.adjoin