Add description of how the directory structure targets Python versions. (#1288)

CONTRIBUTING.md

@@ -170,9 +170,23 @@ contributors.
 
 There are separate directories for `stdlib` and `third_party` stubs.
 Within those, there are separate directories for different versions of
-Python the stubs target.  If the given library works on both Python 2
-and Python 3, prefer to put your stubs in an `2and3` directory, unless
-the types are so different that the stubs become unreadable that way.
+Python the stubs target.
+
+The directory name indicates the major version of Python that a stub targets
+and optionally the lowest minor version, with the exception of the `2and3`
+directory which applies to both Python 2 and 3.
+
+For example, stubs in the `3` directory will be applied to all versions of
+Python 3, though stubs in the `3.6` directory will only be applied to versions
+3.6 and above. However, stubs in the `2` directory will not be applied to
+Python 3.
+
+It is preferred to use a single stub in the more generic directory that
+conditionally targets specific versions when needed, as opposed
+to maintaining multiple stub files within more specific directories. Similarly,
+if the given library works on both Python 2 and Python 3, prefer to put your
+stubs in the `2and3` directory, unless the types are so different that the stubs
+become unreadable that way.
 
 You can use checks like `if sys.version_info >= (3, 4):` to denote new
 functionality introduced in a given Python version or solve type