Merge bitcoindevkit/bdk#1378: Update bdk README

Merge bitcoindevkit/bdk#1378: Update bdk README

06d7dc5c3a890f0a141de4cf6f421fe766d99ec3 doc(bdk): Update bdk README (vmammal)

Pull request description:

  fixes #1044

  ### Notes

  The code snippet is a compile fail because variables `txout` and `outpoint` are not initialized. Any other suggestions are welcome.

  There is still some old commented code in the file that's probably not needed unless we want to provide another example snippet, say for getting new address info.

  * [x] Fix broken links
  * [x] Use a new unused tprv
  * [ ] Try to make persistence example make sense, or maybe just do away with it

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  evanlinjin:
    ACK 06d7dc5c3a890f0a141de4cf6f421fe766d99ec3

Tree-SHA512: 09a671bc6bea574d7a4b42b64718380ee71a0c5d36c6b7ca1dc19a2c567de510b27ccc91fe05e7178bf31c562db66bc64f660415de5bb2f32f369b13c44ad3d2