Show how to use includes in .buckconfig

[carljparker]

• Configuration files can include other configuration files.
• Included files can be specified using relative paths (recommended) or absolute paths from file system root.
• The included files can comprise complete sections or just key/values from a single section. Recommend complete sections only as a best practice.
• The question-mark prefix enables developers to include a file if it exists, but ignore inclusion directive otherwise.
• Documentation at the following URL:
  https://buckbuild.com/files-and-dirs/buckconfig.html

[zpao]