Added minispec-metadata info to readme

[git-p4: depot-paths = "//src/minitest/dev/": change = 8044]

README.txt

@@ -284,6 +284,8 @@ fixture loading:
 == Known Extensions:
 
 capybara_minitest_spec  :: Bridge between Capybara RSpec matchers and MiniTest::Spec expectations (e.g. page.must_have_content('Title')).
+minispec-metadata       :: Metadata for describe/it blocks
+                           (e.g. `it 'requires JS driver', js: true do`)
 minitest-around         :: Around block for minitest. An alternative to setup/teardown dance.
 minitest-capistrano     :: Assertions and expectations for testing Capistrano recipes
 minitest-capybara       :: Capybara matchers support for minitest unit and spec