Clean up credits (python#4626)

In CONTRIBUTING.md, link to CREDITS instead of maintaining a separate
list (which was out of date).

CONTRIBUTING.md

@@ -80,11 +80,7 @@ For every pull request, we aim to promptly either merge it or say why
 it's not yet ready; if you go a few days without a reply, please feel
 free to ping the thread by adding a new comment.
 
-At present the core developers are (alphabetically):
-* David Fisher (@ddfisher)
-* Jukka Lehtosalo (@JukkaL)
-* Greg Price (@gnprice)
-* Guido van Rossum (@gvanrossum)
+For a list of mypy core developers, see the file [CREDITS](CREDITS).
 
 
 Preparing Changes

CREDITS

@@ -7,13 +7,18 @@ https://github.com/python/mypy/commits/master
 For lists of contributors per mypy release (including typeshed) see
 the release blog posts at https://mypy-lang.blogspot.com/.
 
-Mypy team:
+Dropbox core team:
 
   Jukka Lehtosalo <[email protected]>
   Guido van Rossum <[email protected]>
   Ivan Levkivskyi
   Michael J. Sullivan
 
+Non-Dropbox core team members:
+
+  Ethan Smith
+  Jelle Zijlstra
+
 Past Dropbox core team members:
 
   David Fisher
@@ -23,11 +28,6 @@ Past Dropbox core team members:
   Michael Lee
   Reid Barton
 
-Non-Dropbox core team members:
-
-  Ethan Smith
-  Jelle Zijlstra
-
 Additional thanks to:
 
   Alex Allain