Added minitest-stub-const to readme

[git-p4: depot-paths = "//src/minitest/dev/": change = 8151]

README.txt

@@ -324,6 +324,7 @@ minitest-should_syntax  :: RSpec-style +x.should == y+ assertions for MiniTest
 minitest-shouldify      :: Adding all manner of shoulds to MiniTest (bad idea)
 minitest-spec-magic     :: Minitest::Spec extensions for Rails and beyond
 minitest-spec-rails     :: Drop in MiniTest::Spec superclass for ActiveSupport::TestCase.
+minitest-stub-const     :: Stub constants for the duration of a block
 minitest-tags           :: add tags for minitest
 minitest-wscolor        :: Yet another test colorizer.
 minitest_owrapper       :: Get tests results as a TestResult object.