Explained what a deprecation cycle is

doc/contributing.rst

@@ -379,10 +379,31 @@ with ``git commit --no-verify``.
 Backwards Compatibility
 ~~~~~~~~~~~~~~~~~~~~~~~
 
-Please try to maintain backward compatibility. *xarray* has growing number of users with
+Please try to maintain backwards compatibility. *xarray* has a growing number of users with
 lots of existing code, so don't break it if at all possible.  If you think breakage is
-required, clearly state why as part of the pull request.  Also, be careful when changing
-method signatures and add deprecation warnings where needed.
+required, clearly state why as part of the pull request.
+
+Be especially careful when changing function and method signatures, because any change
+may require a deprecation warning. For example, if your pull request means that the
+argument ``old_arg`` to ``func`` is no longer valid, instead of simply raising an error if
+a user passes ``old_arg``, we would instead catch it:
+
+    def func(new_arg, old_arg=None):
+        if old_arg is not None:
+            from warnings import warn
+
+            warn(
+                "`old_arg` has been deprecated, and in the future will raise an error."
+                "Please use `new_arg` from now on."
+            )
+
+            # Still do what the user intended here
+
+This temporary check would then be removed in a subsequent version of xarray.
+This process of first warning users before actually breaking their code is known as a
+"deprecation cycle", and makes changes significantly easier to handle both for users
+of xarray, and for developers of other libraries that depend on xarray.
+
 
 .. _contributing.ci: