Change istructions to include necessary other files.

[fNBU]

Currently the instructions

diff-so-fancy/README.md

Line 13 in 36dcfde

Simply copy the `diff-so-fancy` script from the latest release into your `$PATH` and you're done. Alternately to test development features you can clone this repo and then put the `diff-so-fancy` script (symlink will work) into your `$PATH`. The `lib/` directory will need to be kept relative to the core script.

are a bit confusing. The first sentence says explicitly that only the file diff-so-fancy needs to be copied (which is not correct). The second sentence implies (but does not explicitly say) that the directory lib needs to also be copied to the same location.

This has been mentioned before in eg #318. Regardless of the packaging of a "fatpack" discussed there, the instructions should conform to whatever packaging is used.

[fNBU]

PR: #501