Add a documentation to PDFutils to replace_text and update readme.md

[yodatak]

We should fix the documentiation lopdf/pdfutil/README.md to and and some exemples
USAGE:
pdfutil [OPTIONS] [SUBCOMMAND]

FLAGS:
-h, --help Prints help information
-V, --version Prints version information

OPTIONS:
-i, --input
-o, --output

SUBCOMMANDS:
compress Compress PDF document
decompress Decompress PDF document
delete_objects Delete objects
delete_pages Delete pages
delete_zero_length_streams Delete zero length stream objects
extract_pages Extract pages
extract_stream Extract stream content
extract_text Extract text
help Prints this message or the help of the given subcommand(s)
print_streams Print streams
process Process PDF document with specified operations
prune_objects Prune unused objects
renumber_objects Renumber objects
replace_text Replace text