[readme] add tape-describe to 'other' section (#523)

Closes #522

Author: mattriley

readme.markdown

@@ -136,6 +136,7 @@ By default, uncaught exceptions in your tests will not be intercepted, and will
 - Electron test runner with https://github.com/tundrax/electron-tap
 - Concurrency support with https://github.com/imsnif/mixed-tape
 - In-process reporting with https://github.com/DavidAnson/tape-player
+- Describe blocks with https://github.com/mattriley/tape-describe
 
 # methods