add note about not squashing (#1830)

CONTRIBUTING.md

@@ -75,6 +75,9 @@ processed incorrectly by a type checker. It is also helpful to add
 links to online documentation or to the implementation of the code
 you are changing.
 
+Also, do not squash your commits after you have submitted a pull request, as this
+erases context during review. We will squash commits when the pull request is merged.
+
 At present the core developers are (alphabetically):
 * David Fisher (@ddfisher)
 * Łukasz Langa (@ambv)