Add bindings PGP public key and documentation on how to verify signat…

[thunderbiscuit]

This PR adds our public PGP key to the repository, as well as documentation on how to use it to verify signatures on the bdk-jvm and bdk-android artifacts.

[notmandatory]

I think the "Verify Signatures" section can be simplified as below. Users at a minimum only need to verify the binary artifact signatures match the corresponding .asc file. I don't think they also need to manually check the hashes, checking hashes is done with bitcoin core binaries, but it's a more involved process allowing multiple independent people to provide signatures.

Verify Signatures

Both libraries and all their corresponding artifacts are signed with a PGP key you can find in the
root of this repository. To verify the signatures follow the below steps:

1. Import the PGP key in your keyring.

    # Navigate to the root of the repository and import the ./PGP-BDK-BINDINGS.asc public key
    gpg --import ./PGP-BDK-BINDINGS.asc
    
    # Alternatively, you can import the key directly from a public key server
    gpg --keyserver keyserver.ubuntu.com --receive-key 2768C43E8803C6A3
    
    # Verify that the correct key was imported
    gpg --list-keys
    
    # You should see the below output
    pub   ed25519 2022-08-31 [SC]
          88AD93AC4589FD090FF3B8D12768C43E8803C6A3
    uid           [ unknown] bitcoindevkit-bindings <bindings@bitcoindevkit.org>
    sub   cv25519 2022-08-31 [E]

2. Download the binary artifacts and corresponding signature files.
   • from [bdk-jvm]
     • bdk-jvm-<version>.jar
     • bdk-jvm-<version>.jar.asc
   • from [bdk-android]
     • bdk-android-<version>.aar
     • bdk-android-<version>.aar.asc
3. Verify the signatures.

   gpg --verify bdk-jvm-<version>.jar.asc 
   gpg --verify bdk-android-<version>.aar.asc
   
   # you should see a "Good signature" result
   gpg: Good signature from "bitcoindevkit-bindings <bindings@bitcoindevkit.org>" [unknown]

[thunderbiscuit]

Good ideas. I fixed it all! Ready for re-review.

[thunderbiscuit]

This PR fixes #85.

[notmandatory]

ACK 8e7062c

Thanks for the updates!