Minor readme update

BUG: Fix runtime on invalid UTF-8 encoding

Fixes errors like:

	utf8 "\xC4" does not map to Unicode at ./diff-logs line 52, <$fh1_in> line ...
	Malformed UTF-8 character: \xc4\x2e (unexpected non-continuation byte 0x2e, immediately after start byte 0xc4; need 2 bytes, got 1) in substitution (s///) at ./diff-logs line 59, <$fh1_in> line ...
	Malformed UTF-8 character (fatal) at ./diff-logs line 59, <$fh1_in> line ...

The patch simply assumes UTF-8 and ignores any invalid characters,
replacing them with \xHH escapes.