Add Parallel typeclass diagram to docs.

docs/src/main/tut/typeclasses/parallel.dot

@@ -0,0 +1,34 @@
+digraph parallel {
+  node [shape=none, fontname=courier]
+  edge [fontname=courier]
+
+  label=<Parallel[<font color="red3">M</font>[_]] { type <font color="green3">F</font>[_] }<br/><br/>>
+  labelloc=top
+  fontname="courier-bold"
+
+  subgraph cluster_parallel {
+    label=""
+    color=white
+
+    F [label=<<font color="green3">F</font>[_]>]
+    M [label=<<font color="red3">M</font>[_]>]
+
+    M:ne -> F:nw [label="parallel", fontcolor=green3]
+    F -> M:se [label="sequential", fontcolor=red3]
+
+    { rank=same M F }
+  }
+
+  subgraph cluster_tc {
+    label=""
+    color=white
+
+    Ap [label=<Applicative[<font color="green3">F</font>&#93;>]
+    Monad [label=<Monad[<font color="red3">M</font>&#93;>]
+  }
+
+  F -> Ap [style=dotted dir=none]
+  M -> Monad [style=dotted dir=none]
+
+  { rank=same Monad Ap }
+}

docs/src/main/tut/typeclasses/parallel.md

@@ -74,6 +74,8 @@ trait Parallel[M[_]] {
 ```
 Where `M[_]` has to have an instance of `Monad` and `F[_]` an instance of `Applicative`.
 
+![The `Parallel` typeclass transforms between `Monad` `M[_]` and `Applicative` `F[_]`.](../img/parallel.png)
+
 Recall that `~>` is just an alias for [`FunctionK`](../datatypes/functionk.html).
 This allows us to get rid of most of our boilerplate from earlier: