[d3] Update README.md

[Kelly Tran]

Please update README.md in the /d3/ folder.

• Find the class title + description here (PRO-TIP: search for d3).

• Remove all existing content in README.md.

• Add the following to README.md:

# {{ CLASS_TITLE }}

where {{ CLASS_TITLE }} is the class title found in the google doc above.

• Add the class description found in the google doc above.

• Check that bolded text, italic text, lists, links are properly formatted.

[oxleberry]

I've got this!