Add CLI Example to Readme (#909)

Simple change to add a real world example of CLI usage, because it wasn't immediately obvious to me how it should actually look

README.md

@@ -113,10 +113,16 @@ 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.
 
-```js
+```bash
 prettier [opts] [filename ...]
 ```
 
+In practice, this may look something like:
+
+```bash
+prettier --single-quote --trailing-comma es5 --write '{app,__{tests,mocks}__}/**/*.js'
+```
+
 For easier cross-platform usage, prettier has built-in glob support:
 ```bash
 prettier --write "src/**/*.js" "bin/*.js"