Add --stdin-filepath to website (#4513)

docs/cli.md

@@ -115,3 +115,20 @@ Change the level of logging for the CLI. Valid options are:
 - `log` (default)
 - `debug`
 - `silent`
+
+## `--stdin-filepath`
+
+A path to the file that the Prettier CLI will treat like stdin. For example:
+
+_abc.css_
+```css
+.name { display: none; }
+```
+
+_shell_
+```bash
+$ cat abc.css | prettier --stdin-filepath abc.css
+.name {
+  display: none;
+}
+```