Add a description of the 311 service requests dataset in Chapter 2

Author: jvns

cookbook/Chapter 2 - Selecting data & finding the most common complaint type.ipynb

@@ -29,7 +29,7 @@
      "cell_type": "markdown",
      "metadata": {},
      "source": [
-      "We're going to continue with the same example from Chapter 1, to start. "
+      "We're going to use a new dataset here, to demonstrate how to deal with larger datasets. This is a subset of the of 311 service requests from [NYC Open Data](https://nycopendata.socrata.com/Social-Services/311-Service-Requests-from-2010-to-Present/erm2-nwe9). "
      ]
     },
     {
@@ -261,7 +261,9 @@
      "cell_type": "markdown",
      "metadata": {},
      "source": [
-      "To get the first 5 rows of a dataframe, we can use a slice: `df[:5]`. If the dataframe has too many "
+      "To get the first 5 rows of a dataframe, we can use a slice: `df[:5]`.\n",
+      "\n",
+      "This is a great way to get a sense for what kind of information is in the dataframe -- take a minute to look at the contents and get a feel for this dataset."
      ]
     },
     {