This article is from the PGP FAQ, by Jeff Licquia firstname.lastname@example.org with numerous contributions by others.
Sometimes you are not interested in keeping the contents of a message
secret, you only want to make sure that nobody tampers with it, and to
allow others to verify that the message is really from you. For this,
you can use clear signing. Clear signing only works on text files, it
will NOT work on binary files. The command format is:
pgp -sat +clearsig=on <filename>
The output file will contain your original unmodified text, along with
section headers and an armored PGP signature. In this case, PGP is not
required to read the file, only to verify the signature.