Merge bitcoindevkit/bdk#1661: chore(file_store): Document that it's a dev database

4b5bd0b14bfba95f0ca4e2c612d6e24832ba5b73 chore(file_store): Document that it's a dev database (LLFourn)

Pull request description:

  Document `bdk_file_store` is a development/testing database

ACKs for top commit:
  LagginTimes:
    ACK 4b5bd0b14bfba95f0ca4e2c612d6e24832ba5b73
  ValuedMammal:
    ACK 4b5bd0b14bfba95f0ca4e2c612d6e24832ba5b73
  oleonardolima:
    ACK 4b5bd0b14bfba95f0ca4e2c612d6e24832ba5b73

Tree-SHA512: a374753fe3d367870afc7b4cb301ef7fd09da1babefb0601191b266123358c2ffd247e4c6c5fb84eaa0e3b2821f8c0b1337cde6d63bbc2dce3fb7d5ff0286cef