[lib][unittest] Add docs on use of printf
ZX-2218 #comment patch
Change-Id: If12efb6d0cc90e250841731a3ec07cf1de61ddf0
diff --git a/system/ulib/unittest/README.md b/system/ulib/unittest/README.md
index bea5715..caba78d 100644
--- a/system/ulib/unittest/README.md
+++ b/system/ulib/unittest/README.md
@@ -9,6 +9,23 @@
**If you want your unit tests to print to standard out, you must link your
test executable to system/ulib/fdio!**
+## Rules for use of printf vs unittest_printf
+
+Tests are expected to *not* call `printf()`. By default we want tests
+to be less verbose, and use of printfs about means the output cannot
+not be disabled. This test harness can call `printf()`, but tests should not.
+Instead tests are expected to either call `unittest_printf()` or
+`unitest_printf_critical()`, the difference being that the former is
+controlled by the verbosity level, and the latter is not.
+Obviously `unittest_printf_critical()` is intended to be used *sparingly*.
+
+## Test verbosity
+
+Verbosity is controlled by passing `v=N` when invoking the test,
+where N has the range 0-9. The default is zero, which means
+`unittest_printf()` output does not appear. Any value above zero enables
+`unittest_printf()` output.
+
## Rules for parsing argv
Unittest has a set of options that it recognizes.
