Documenting require.main, fixing #997

doc/api/modules.markdown

@@ -330,6 +330,21 @@ Because `module` provides a `filename` property (normally equivalent to
 `__filename`), the entry point of the current application can be obtained
 by checking `require.main.filename`.
 
+### Accessing the main module
+
+When a file is run directly from Node, `require.main` is set to its
+`module`. That means that you can determine whether a file has been run
+directly by testing
+
+    require.main === module
+
+For a file `foo.js`, this will be `true` if run via `node foo.js`, but
+`false` if run by `require('./foo')`.
+
+Because `module` provides a `filename` property (normally equivalent to
+`__filename`), the entry point of the current application can be obtained
+by checking `require.main.filename`.
+
 ## Addenda: Package Manager Tips
 
 The semantics of Node's `require()` function were designed to be general