[3.6] bpo-28929: Add to Misc/NEWS (GH-284)

mention bpo-28929 in the Documentation section of
What's New in Python 3.6.1 release candidate 1
diff --git a/Misc/NEWS b/Misc/NEWS
index d885ca9..0f5c279 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -217,6 +217,8 @@
 Documentation
 -------------
 
+- bpo-28929: Link the documentation to its source file on GitHub.
+
 - bpo-25008: Document smtpd.py as effectively deprecated and add a pointer to
   aiosmtpd, a third-party asyncio-based replacement.