write amount function tutorial

docs/usage/growth/amount.rst

@@ -1,3 +1,39 @@
 ========================
 Amount Functions
-========================
+========================
+
+Although we have introduced the familiar cases of simple and compound interest, not all growth patterns are linear or geometric. Sometimes a growth pattern might be geometric, cubic, or some arbitrary user-defined pattern.
+
+To accommodate these new patterns, we can define an **amount function**, which specifies how money grows for an arbitrary growth pattern:
+
+.. math::
+   A_K(t)
+
+Where :math:`K` specifies the amount of principal, :math:`t` specifies the amount of time, and :math:`A_K(t)` returns the value at time :math:`t` of :math:`K` invested at time 0.
+
+Examples
+========================
+
+Suppose money exhibits a quadratic growth pattern, specified by the amount function:
+
+.. math::
+   A_K(t) = K(.05t^2 + .05t + 1)
+
+If we invest :math:`K=5` at time 0, how much does it grow to at time 5?
+
+TmVal's *Amount* class allows us to model this behavior. To solve the above problem, simply call the class and supply the growth function and principal. First, define the growth function as a Python function that takes the time and principal as arguments:
+
+.. ipython:: python
+
+   from tmval import Amount
+
+   def f(t, k):
+       return k * (.05 * (t **2) + .05 * t + 1)
+
+Now supply the growth_function to the Amount class, and call :code:`my_amt.val(5)` to get the answer:
+
+.. ipython:: python
+
+   my_amt = Amount(f=f, k=5)
+
+    print(my_amt.val(5))