ripgrep: improve usage documentation

This shows an example for reading stdin.

Fixes #951

Author: BurntSushi

CHANGELOG.md

@@ -84,6 +84,8 @@ Bug fixes:
 * [BUG #948](https://github.com/BurntSushi/ripgrep/issues/948):
   ripgrep now uses an exit code of 2 to indicate an error, and uses an exit
   code of 1 to indicate that no matches were found.
+* [BUG #951](https://github.com/BurntSushi/ripgrep/issues/951):
+  Add stdin example to ripgrep usage documentation.
 * [BUG #955](https://github.com/BurntSushi/ripgrep/issues/955):
   Use buffered writing when not printing to a tty, which fixes a performance
   regression.

doc/rg.1.txt.tpl

@@ -18,6 +18,8 @@ Synopsis
 
 *rg* [_OPTIONS_] *--type-list*
 
+*command* | *rg* [_OPTIONS_] _PATTERN_
+
 *rg* [_OPTIONS_] *--help*
 
 *rg* [_OPTIONS_] *--version*

src/app.rs

@@ -33,7 +33,8 @@ const USAGE: &str = "
     rg [OPTIONS] PATTERN [PATH ...]
     rg [OPTIONS] [-e PATTERN ...] [-f PATTERNFILE ...] [PATH ...]
     rg [OPTIONS] --files [PATH ...]
-    rg [OPTIONS] --type-list";
+    rg [OPTIONS] --type-list
+    command | rg [OPTIONS] PATTERN";
 
 const TEMPLATE: &str = "\
 {bin} {version}