[Distutils] PEP 427 (wheel) clarifications

[Daniel Holth]

A few more clarifications to PEP 427 "wheel".

+    The ``b'#!pythonw'`` convention is allowed. ``b'#!pythonw'`` indicates
+    a GUI script instead of a console script.
+

...

     { "hash": "sha256=ADD-r2urObZHcxBW3Cr-vDCu5RJwT4CaRTHiFmbcIYY" }

+(The hash value is the same format used in RECORD.)
+

+What's the deal with "purelib" vs. "platlib"?
+    Wheel preserves the historic "purelib" vs. "platlib" distinction
+    even though both map to the same install location in any system the
+    author could find.
+
+    For example, a wheel with "Root-Is-Purelib: false" with all its files
+    in ``{name}-{version}.data/purelib`` is equivalent to a wheel with
+    "Root-Is-Purelib: true" with those same files in the root, and it
+    is legal to have files in both the "purelib" and "platlib" categories.
+
+    In practice a wheel should have only one of "purelib" or "platlib"
+    depending on whether it is pure Python or not and those files should
+    be at the root.