Document the feature of input from stdin

Since #329, rgbasm can get input from stdin. I updated the manpage
file related to it, which explain the description and usages in
rgbasm.

CONTRIBUTORS.rst

@@ -45,3 +45,5 @@ Other contributors
 - yenatch <yenatch@gmail.com>
 
 - phs <phil@philhsmith.com>
+
+- jidoc01 <jidoc01@naver.com>

src/asm/rgbasm.1

@@ -1,11 +1,11 @@
 .\"
 .\" This file is part of RGBDS.
 .\"
-.\" Copyright (c) 2010-2018, Anthony J. Bentley and RGBDS contributors.
+.\" Copyright (c) 2010-2019, Anthony J. Bentley and RGBDS contributors.
 .\"
 .\" SPDX-License-Identifier: MIT
 .\"
-.Dd February 24, 2018
+.Dd July 8, 2019
 .Dt RGBASM 1
 .Os RGBDS Manual
 .Sh NAME
@@ -26,6 +26,12 @@
 The
 .Nm
 program creates an object file from an assembly source file.
+The input
+.Ar file
+can be a file path, or
+.Cm \-
+denoting
+.Cm stdin .
 Its arguments are as follows:
 .Bl -tag -width Ds
 .It Fl b Ar chars
@@ -79,12 +85,20 @@ Be verbose.
 Disable warning output.
 .El
 .Sh EXAMPLES
-Assembling a basic source file is simple:
+You can assemble a source file in two ways.
+Straight forward way:
 .Pp
 .Bd -literal -offset indent
 $ rgbasm -o bar.o foo.asm
 .Ed
 .Pp
+Pipes way:
+.Pp
+.Bd -literal -offset indent
+$ cat foo.asm | rgbasm -o bar.o -
+$ rgbasm -o bar.o - < foo.asm
+.Ed
+.Pp
 The resulting object file is not yet a usable ROM image \(em it must first be
 run through
 .Xr rgblink 1