Add reference to existing Medium articles and YouTube posts to main README

[andrewrfreed]

At minimum link to this 3-part blog (w/ embedded video) series:

Type	Title	Link
Blog	Testing a Chatbot with k-folds Cross Validation	https://medium.com/ibm-watson/testing-a-chatbot-with-k-folds-cross-validation-68dab111a6b
Video	Testing a Chatbot with k-folds Cross Validation	https://www.youtube.com/watch?v=FrhK68WyOK4
Blog	Analyze chatbot classifier performance from logs	https://medium.com/ibm-watson/analyze-chatbot-classifier-performance-from-logs-e9cf2c7ca8fd
Video	Analyze chatbot classifier performance from logs	https://www.youtube.com/watch?v=yd89DKyf6hc
Blog	Improve a chatbot classifier with production data	https://medium.com/ibm-watson/improve-a-chatbot-classifier-with-production-data-22a437f419b4
Video	Improve a chatbot classifier with production data	https://www.youtube.com/watch?v=ftFIQtHiQY8