Document mypy exit codes

docs/source/exit_codes.rst

@@ -0,0 +1,20 @@
+.. _exit-codes:
+
+Exit codes
+===========
+
+Mypy, like all programs, returns an
+`exit code <https://en.wikipedia.org/wiki/Exit_status>`_ to the process or shell
+that invokes it. The meaning of these codes is thus:
+
+* 0: no type errors
+
+* 1: there are some type errors
+
+* 2: a crash, bad arguments or configuration, and other non-standard conditions relating to mypy
+
+There is currently `a bug <https://github.com/python/mypy/issues/19548>`_
+in which certain configuration errors will result in an exit code of 0 instead
+of 2.
+
+Note: the exit code is completely unrelated to mypy-internal :ref:`error-codes`.

docs/source/index.rst

@@ -101,6 +101,7 @@ Contents
    error_codes
    error_code_list
    error_code_list2
+   exit_codes
    additional_features
    faq
    changelog