Proposal: add sample code for transitions into README docs

[curran]

In order to make the documentation in the README more complete for people looking for the appropriate pattern to use d3.join with transitions, I suggest to add the following snippet directly inside the README, so that one may study it without leaving the page:

    svg.selectAll("text")
      .data(randomLetters(), d => d)
      .join(
        enter => enter.append("text")
            .attr("fill", "green")
            .attr("x", (d, i) => i * 16)
            .attr("y", -30)
            .text(d => d)
          .call(enter => enter.transition(t)
            .attr("y", 0)),
        update => update
            .attr("fill", "black")
            .attr("y", 0)
          .call(update => update.transition(t)
            .attr("x", (d, i) => i * 16)),
        exit => exit
            .attr("fill", "brown")
          .call(exit => exit.transition(t)
            .attr("y", 30)
            .remove())
      );

For what it's worth, I personally find this to be a most useful reference that I look to repeatedly. It would be great to skip the steps of clicking through to the Observable notebook, and scrolling down that page to search for this snippet. Thanks!

[mbostock]

This is covered by #284, so closing.