Merge bitcoindevkit/bdk#1573: chore: add missing bdk_core README.md and remove specif...

48b6a66f6e0edce305d41653f37b8549aaf7c060 chore(core): remove bdk_chain dev-dependency version (Steve Myers)
9bd500f5f198df4dfc72a1f71c93f9b20d65431d chore(core): add missing README.md (Steve Myers)

Pull request description:

  Add the missing README.md file for the bdk_core crate. This is required to publish to crates.io.

  Also had to removed fixed bdk_chain version from bdk_core dev dependencies to prevent missing dependency when publishing bdk_core before publishing new version of bdk_chain.

ACKs for top commit:
  ValuedMammal:
    ACK 48b6a66f6e0edce305d41653f37b8549aaf7c060

Tree-SHA512: 5fb31984f79771c5366ca078efa11330b24fd4d97e90531a2ba8f92ce6361c6dd4f123e8640269de9235bc5ebf3e6b5ced261e36f5b40f01db3f82add9a5ae68