Clarify that we only support GNU diff

In issue #57, it was reported that the library could not apply some
patches generated by BSD `diff` because of the way that variant reports
changes in files without trailing newlines. Since the library behavior
matches Git, I don't consider this a bug, but update the README to
mention that the support for standard unified diffs is only for the GNU
variant of the `diff` tool.

README.md

···
 
 A Go library for parsing and applying patches generated by `git diff`, `git
 show`, and `git format-patch`. It can also parse and apply unified diffs
-generated by the standard `diff` tool.
+generated by the standard GNU `diff` tool.
 
 It supports standard line-oriented text patches and Git binary patches, and
 aims to parse anything accepted by the `git apply` command.