Mention how stdio handling works

Fixes #57

README.md

@@ -54,6 +54,12 @@ Options:
   --help                      Show this message and exit.
 ```
 
+`Black` is a well-behaved Unix-style command-line tool:
+* it does nothing if no sources are passed to it;
+* it will read from standard input and write to standard output if `-`
+  is used as the filename;
+* it only outputs messages to users on standard error.
+
 
 ## The philosophy behind *Black*
 
@@ -209,6 +215,12 @@ body.
 
 * Visual Studio Code: [joslarson.black-vscode](https://marketplace.visualstudio.com/items?itemName=joslarson.black-vscode)
 
+Any tool that can pipe code through *Black* using its stdio mode (just
+[use `-` as the file name](http://www.tldp.org/LDP/abs/html/special-chars.html#DASHREF2)).
+The formatted code will be returned on stdout (unless `--check` was
+passed).  *Black* will still emit messages on stderr but that shouldn't
+affect your use case.
+
 There is currently no integration with any other text editors. Vim and
 Atom/Nuclide integration is planned by the author, others will require
 external contributions.