Updating file format in README.
Updating file format in README.

Updating file format to remove the use of CRC32
checksums for content, resuming SHA1 digests in
mid calculations (not supported by OpenSSL), using
sha1 digests for chunks of data (for use when
determining which part of file is corrupt), and
grouping data related to original file together.
bindle/filehatchet,bindle/filehatchet