Add rank cookbook recipes. Closes #110. Closes #160.

docs/cookbook/ranking.rst

@@ -2,7 +2,34 @@
 Ranking
 =======
 
-Ranking is a complicated science. agate strives to find a balance between simple, intuitive ranking and flexibility when you need it.
+There are many ways to rank a sequence of values. agate strives to find a balance between simple, intuitive ranking and flexibility when you need it.
+
+Competition rank
+================
+
+The basic rank supported by agate is standard "competition ranking". In this model the values :code:`[3, 4, 4, 5]` would be ranked :code:`[1, 2, 2, 4]`. You can apply competition ranking using the :class:`.Rank` computation:
+
+.. code-block:: python
+
+    new_table = table.compute([
+        ('rank', agate.Rank('value')
+    ])
+
+Rank change
+===========
+
+You can compute the change from one rank to another by combining the :class:`.Rank` and class:`.Change` computations:
+
+.. code-block:: python
+
+    new_table = table.compute([
+        ('rank2014', agate.Rank('value2014'),
+        ('rank2015', agate.Rank('value2015')
+    ])
+
+    new_table2 = table.compute([
+        ('rank_change', agate.Change('rank2014', 'rank2015')
+    ])
 
 Percentile rank
 ===============
@@ -11,10 +38,8 @@ Percentile rank
 
 .. code-block:: Python
 
-    from agate import PercentileRank
-
     new_table = table.compute([
-        ('percentile_rank', PercentileRank('value')
+        ('percentile_rank', agate.PercentileRank('value')
     ])
 
 Note that there is no entirely standard method for computing percentiles. The percentiles computed in this manner may not agree with those generated by other programes. See the :class:`.Percentiles` class documentation for implementation details.