Add license notice to regex_syntax.rs

The comment in the code says this code was copied from another source
with an appropriate link. However just local to this file it is not
clear what the license terms of that code are. I added the license
notice to this file that governs the particular code that was copied
over.

I would like to use this crate in an environment where we want to be
clear about licensing. Hopefully this is an okay change otherwise I
would be more than happy to do this a different way as long as the
license provenance is clear.

schemars_derive/src/regex_syntax.rs

@@ -1,5 +1,31 @@
 // Copied from regex_syntax crate to avoid pulling in the whole crate just for a utility function
 // https://github.com/rust-lang/regex/blob/ff283badce21dcebd581909d38b81f2c8c9bfb54/regex-syntax/src/lib.rs
+//
+// Copyright (c) 2014 The Rust Project Developers
+//
+// Permission is hereby granted, free of charge, to any
+// person obtaining a copy of this software and associated
+// documentation files (the "Software"), to deal in the
+// Software without restriction, including without
+// limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of
+// the Software, and to permit persons to whom the Software
+// is furnished to do so, subject to the following
+// conditions:
+//
+// The above copyright notice and this permission notice
+// shall be included in all copies or substantial portions
+// of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+// ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+// TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+// PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+// SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+// CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+// IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
 
 pub fn escape(text: &str) -> String {
     let mut quoted = String::new();