add validation method, data leaks

fix links

test markdown

add notes on fillna and feature extraction

add pratical notes: feature engineering

add feature preprocessing methods

add t-SNE

decide num of clustering in Unsupervised learning

add notes on clustering

Remove practice follow Coursera rule

modified readme.md and rename slides file name

adding week11 notes

adding lecture18 slides all done

add notes on Large scale ML

adding lecture 17 slides