[build] Review license management

[sarod]

• Create a proper LICENSE file
• Create a CONTRIBUTORS file
• Simplify the file license header in files

License header are not always to date and should be simplified to the minimum to avoid repeating ourself.
According to some it seems that the copyright file header may be totally obsolete. However it seems to be common practice to have one just as a reminder for people browsing code that their is a License.

It could be as simple as that.

/**
 * Copyright (c) Infinitest copyright holders. 
 * See LICENSE file in the project root for full license information. 
 */

Add a CONTRIBUTORS.md file at the root of their repository that lists the authors and copyright holders that have made non-trivial contributions to that project source code.

Note that this is inspired by https://forums.dotnetfoundation.org/t/file-headers-and-copyright-statements/1276