Merge bitcoindevkit/bdk#1082: docs: add License and Contributing sections to repo...

Merge bitcoindevkit/bdk#1082: docs: add License and Contributing sections to repo top-level README

2d543475e2d7aa5503199699bb9bb4c0e519d0a8 docs: add License and Contributing sections to repo top-level README (Steve Myers)

Pull request description:

  ### Description

  Add explicit "License" and "Contributing" sections to the repo top-level `README.md` file to make it clear that it applies to everything in the project. Also moved license files up to repo top-level directory and did some minor cleanup of bdk/README.md License and Contribution sections to match top-level README.

ACKs for top commit:
  danielabrozzoni:
    ACK 2d543475e2d7aa5503199699bb9bb4c0e519d0a8

Tree-SHA512: 00442adf22258333d7672e8389b2380a8a81219f217bbffd525e253424a2aeeed13dd2316b3fe69d0bbeb13addbf2351697659c2abfec762e8e789fce3bb26e2