Added environment variable documentation

doc/env.rst

@@ -0,0 +1,15 @@
+Environment and CLI
+===================
+
+Tests built with Criterion support environment variables to alter
+their runtime behaviour.
+
+Environment Variables
+---------------------
+
+* `CRITERION_ALWAYS_SUCCEED`: when set to `1`, the exit status of the test
+  process will be 0, regardless if the tests failed or not.
+* `CRITERION_NO_EARLY_EXIT`: when set to `1`, the test workers shall not
+  call `_exit` when done and will properly return from the main and
+  clean up their process space. This is useful when tracking memory leaks with
+  `valgrind --tool=memcheck`.

doc/index.rst

@@ -8,3 +8,4 @@ Criterion
     setup
     starter
     hooks
+    env