Document how to use the --glob parameter

This makes it easier to debug a single page, especially when it fails to
build.

README.md

@@ -194,3 +194,29 @@ or to execute the last two commands just run:
 ```sh
 bin/run
 ```
+
+## Building a single file
+
+If you only want Middleman to build a single file, you can do that via the `--glob` parameter.
+Here are a few things to keep in mind:
+
+* The glob parameter must match the **destination** file.
+
+* The `--no-clean` option should be included or Middleman will wipe out
+  files that do not match the glob parameter.
+
+For example, here's how to rebuild the Contribute page. Note how
+`source/company/culture/index.html.md.erb` is mapped to
+`company/culture/contribute/index.html`:
+
+```sh
+bundle exec middleman build --glob={company/culture/contribute/index.html} --no-clean
+```
+
+For blog posts,
+`source/posts/2017-05-23-attributes-of-successful-development-teams.html.md`
+maps to `2017/05/23/attributes-of-successful-development-teams/index.html`:
+
+```sh
+bundle exec middleman build --glob={2017/05/23/attributes-of-successful-development-teams/index.html} --no-clean
+```