added example 3 to docs

docs/source/examples/example_3_turbine_type_optimization.rst

+.. Example 3
+
+Example 3: Turbine Type Optimization
+===================================================
+
+This example uses a dummy cost function to optimize a simple wind turbine
+layout that is subject to constraints. The optimization pushes the wind turbine
+locations to specified locations in the farm.
+
+Specifications
+--------------
+- This optimization uses the FullFactorialGenerator from openmdao's doe_generators
+
+Results
+-------
+
+The optimization results are visualized in the below GIF.  
+
+.. figure:: /../../examples/example_3_turbine_type_optimization.gif
+
+Code
+----
+
+.. literalinclude:: /../../examples/example_3_turbine_type_optimization.py

docs/source/index.rst

@@ -17,3 +17,4 @@ Welcome to TOPFARM, the wind-farm optimizer
 
     examples/example_1_constrained_layout_optimization
     examples/example_2_wake_comparison
+    examples/example_3_turbine_type_optimization