Give width and height in Getting Started code in README.md

[rousan]

Use width and height in Getting started code in Readme.md file as follows:

  canvas
  .data(dm) // Set data to the chart.
  .rows(["Horsepower"]) // Fields drawn on Y axis.
  .columns(["Origin"]) // Fields drawn on X axis.
  .width(600)
  .height(700)
  .mount("#chart"); // Specify an element to mount on using a CSS selector.