added option -X

README

@@ -65,7 +65,38 @@ QUICKSTART
     just sick of Alice and Bob. We're running NSA-free, so we're using other
     sample names as well.
 
-INSTALLATION
+FILES AND PIPES
+    Pcp behaves like any other unix tool. If not otherwise specified it will
+    read input from standard input (STDIN) and print output to standard
+    output (STDOUT). For instance:
+
+     pcp1 -e -O output
+
+    will read the text to be encrypted from standard input, because -I has
+    not been specified. It works the same with -O:
+
+     pcp1 -e -I myfile
+
+    In this case the encrypted result will be written to standard output.
+
+    Therefore it is possible to use pcp within pipes. Another more realistic
+    example:
+
+     ssh remote cat file | pcp1 -ez | mailx -s 'as requested' bob@somewhere
+
+    here we encrypt a file symmetrically without downloading it from a
+    remote ssh server and sending the encrypted result via email to someone.
+
+    The behavior is the same with any other functionality where files are
+    involved like importing or exporting keys. However, there's one
+    exception: If the option -X (--password-file) has been used and is set
+    to -, then this will take precedence over any other possible use of
+    standard input. So if you want to encrypt something and don't specify an
+    input file you cannot use -X -, and vice versa. IF you use -X - the
+    passphrase will be read from standard input, which then can't be used
+    further for input files elsewhere. Pcp will exit with an error in such a
+    case. =head1 INSTALLATION
+
     There are currently no packages available, so pcp has to be compiled
     from source. Follow these steps: