Update PEP 562 with an example (#418)

This adds an example of module __getattr__ usage that implements lazy imports to Rationale of PEP 562.

pep-0562.rst

@@ -47,6 +47,31 @@ on module *instances*. For example::
 
   from lib import old_function  # Works, but emits the warning
 
+Another widespread use case for ``__getattr__`` would be lazy submodule
+imports. Consider a simple example::
+
+  # lib/__init__.py
+
+  import importlib
+
+  __all__ = ['submod', ...]
+
+  def __getattr__(name):
+      if name in __all__:
+          return importlib.import_module("." + name, __name__)
+      raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
+
+  # lib/submod.py
+
+  print("Submodule loaded")
+  class HeavyClass:
+      ...
+
+  # main.py
+
+  import lib
+  lib.submodule.HeavyClass  # prints "Submodule loaded"
+
 There is a related proposal PEP 549 that proposes to support instance
 properties for a similar functionality. The difference is this PEP proposes
 a faster and simpler mechanism, but provides more basic customization.