guide: describe the how hash values (`md5`) are calculated

[efiop]

Tell about how we compute checksums for files, why we dos2unix when computing md5 and how to map the md5 value found in dvcfile to an actual cache file at .dvc/cache

[algomaster99]

@efiop @shcheklein Can I take this issue? Also, please provide me with the link of the file where this evaluation happens 😄

[shcheklein]

discussed in private, will start with #271 for now.

[jorgeorpinel]

Lets make sure to explain that "On windows dvc is calculating the md5sum of text files with the windows \r\n line endings replaced with unix \n line endings." per this message.

Nevermind, it seems that will no longer be the case in the near future per this msg.

[jorgeorpinel]

More details from Ruslan

Our checksums usually match with md5/md5sum (commands), except in cases where you have text files with windows-style crlf line endings.