diff usage reminder

Hint: to make life easy, just use diff -u to produce unified format with left margin +/- symbols

may not be a good move for large files/many changes, in which case the terser standard format is explained below for reference
by convention (Wiki), the first file argument should be the original, and the second should be the new version
first file argument is the one listed above the --- fold, the second below it
in this context, an addition/insertion is represented with a < and a removal/deletion with >
- < above the fold = "has come in" (has replaced what's below the fold)
- > below the fold = "has gone out" (has been replaced by what's below the fold)
A diff call will not necessarily give the same numbers of lines above and below the fold with the file arguments passed in a different order!

Interpreting `diff` output codes