Add mandoc-manpage for cat(1)

and mark it as finished in the README.

README

@@ -10,7 +10,7 @@ The following tools are implemented (* == finished):
   -------         --------------------            ---------------
 * basename        yes                             none
   cal             pending                         pending
-  cat             yes                             none
+* cat             yes                             none
   chgrp           no                              -h, -H, -L, -P
   chmod           yes                             none
   chown           no                              -h, -H, -L, -P

cat.1

@@ -1,10 +1,29 @@
-.TH CAT 1 sbase\-VERSION
-.SH NAME
-cat \- concatenate files
-.SH SYNOPSIS
-.B cat
-.RI [ file ...]
-.SH DESCRIPTION
-.B cat
-reads each file in sequence and writes it to stdout.  If no file is given, cat
+.Dd January 16, 2015
+.Dt CAT 1 sbase\-VERSION
+.Sh NAME
+.Nm cat
+.Nd concatenate files
+.Sh SYNOPSIS
+.Nm cat
+.Op Fl u
+.Op Ar file ...
+.Sh DESCRIPTION
+.Nm
+reads each
+.Ar file
+in sequence and writes it to stdout. If no
+.Ar file
+is given,
+.Nm
 reads from stdin.
+.Sh OPTIONS
+.Bl -tag -width Ds
+.It Fl u
+Unbuffered output
+.El
+.Sh STANDARDS
+The
+.Nm
+utility is compliant with the
+.St -p1003.1-2008
+specification.