add documentation for simple interest use case

docs/usage/growth/simple.rst

@@ -0,0 +1,60 @@
+========================
+Simple Interest
+========================
+Simple interest is a pattern of money growth in which the value of money increases at a linear rate:
+
+.. math::
+
+   a(t) = 1 + st
+
+where :math:`a(t)` refers to the value of 1 dollar (or other unit of currency) after time :math:`t`, at interest rate `s`. For example, $1 that grows at 5% simple interest is expected to grow to $1.05 after 1 year:
+
+.. math::
+
+   a(1) = 1 + (.05)(1) = 1.05.
+
+For quantities of money larger than dollar, we can express growth as:
+
+.. math::
+
+   A_K(t) = K(1 + st)
+
+Where :math:`K` refers to the initial amount, or **principal**. For example, if we start with $5 and an interest rate of 5%, it should grow to $5.25 after one year:
+
+.. math::
+
+   A_K(1) = 5(1 + (.05)(1)) = 5.25
+
+Examples
+========================
+
+Let's repeat the above examples using the TmVal package. Let's start by importing SimpleAmt, which is a class that can be used for simple interest calculations:
+
+.. ipython:: python
+
+   from tmval import SimpleAmt
+
+Let's see how much $1 grows to after 1 year, at an interest rate of 5%:
+
+.. ipython:: python
+
+   my_amt = SimpleAmt(k=1, s=.05)
+   print(my_amt.val(1))
+
+
+Now, let's change the principal to $5:
+
+.. ipython:: python
+
+   my_amt = SimpleAmt(k=5, s=.05)
+   print(my_amt.val(1))
+
+The output is 5.25, the same as above.
+
+TmVal also comes with a simple interest solver that can be used to solve for missing inputs. For example, what rate of interest would give us $5.25, if we held $5 for a year?
+
+.. ipython:: python
+
+   from tmval import get_simple_amt
+   my_amt = get_simple_amt(fv=5.25, interest=.05, n=1)
+   print(my_amt.interest_rate)