docs: add globbing example

Although prettier lacks options to filter project files to format by itself, most shells allow to submit multiple files with a filename expansion. Add an example using bash.

README.md

@@ -97,13 +97,19 @@ Run prettier through the CLI with this script. Run it without any
 arguments to see the options.
 
 To format a file in-place, use `--write`. While this is in beta you
-should probably commit your code before doing that. In the future we
-will have better support for formatting whole projects.
+should probably commit your code before doing that.
 
 ```js
-prettier <opts> <filename>
+prettier [opts] [filename ...]
 ```
 
+For example, you could format your source using bash filename expansion:
+```bash
+prettier --write src/**/*.js bin/*.js
+```
+
+In the future we will have better support for formatting whole projects.
+
 ### API
 
 The API is a single function exported as `format`. The options