doc: add copyright headers

[muxator]

This PR:

• adds a copyright header in the frost module, in line with what already exists - for example - for the schnorrsig module:
  

  secp256k1/src/modules/schnorrsig/main_impl.h

  Lines 1 to 5 in 5b32602

  	/***********************************************************************
  	* Copyright (c) 2018-2020 Andrew Poelstra, Jonas Nick *
  	* Distributed under the MIT software license, see the accompanying *
  	* file COPYING or https://www.opensource.org/licenses/mit-license.php.*
  	***********************************************************************/

  
  This is the header that gets added:

  /***********************************************************************
   * Copyright (c) 2023 Bank of Italy                                    *
   * Distributed under the MIT software license, see the accompanying    *
   * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
   ***********************************************************************/
  

• modifies the READMEs to mention that the original author of the library is Bank of Italy.

[muxator]

Wrong repo, my bad.

[real-or-random]

By the way, there's another fork of this library, which has a PR for a FROST module.