doc/terml: More doc.

Author: MostAwesomeDude

doc/terml.rst

@@ -29,3 +29,20 @@ That's it! We've created an empty term, `Term`, with nothing inside.
 We can see that terms are not just `namedtuple` lookalikes. They have their
 own internals and store data in a slightly different and more structured way
 than a normal tuple.
+
+Parsing Terms
+=============
+
+Parsley can parse terms from streams. Terms can contain any kind of parseable
+data, including other terms. Returning to the ubiquitous calculator example::
+
+    add = Add(:x, :y) -> x + y
+
+Here this rule matches a term called `Add` which has two components, bind
+those components to a couple of names (`x` and `y`), and return their sum. If
+this rule were applied to a term like `Add(3, 5)`, it would return 8.
+
+Terms can be nested, too. Here's an example that performs a slightly contrived
+match on a negated term inside an addition::
+
+    add_negate = Add(:x, Negate(:y)) -> x - y