doc: note ability to require files within packages

Adding note to api/modules.markdown about ability to
load arbitrary path from modules from node_module path
by adding reference to path after module name in call
to require()

PR-URL: #7794
Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-by: Fedor Indutny <fedor@indutny.com>
Reviewed-by: Timothy J Fontaine <tjfontaine@gmail.com>

doc/api/modules.markdown

@@ -175,6 +175,12 @@ this order:
 This allows programs to localize their dependencies, so that they do not
 clash.
 
+You can require specific files or sub modules distributed with a module by
+including a path suffix after the module name. For instance
+`require('example-module/path/to/file')` would resolve `path/to/file`
+relative to where `example-module` is located. The suffixed path follows the
+same module resolution semantics.
+
 ## Folders as Modules
 
 <!--type=misc-->