Adding more C# examples

Author: jbw3

README.md

@@ -517,13 +517,74 @@ which may be called at startup without having acquired the GIL.
 
 When finished using Python APIs, managed code must call a corresponding
 `PythonEngine.ReleaseLock` to release the GIL and allow other threads
-to use Python.
+to use Python:
+
+```csharp
+IntPtr gilState = PythonEngine.AcquireLock();
+
+PythonEngine.Exec("doStuff()");
+
+PythonEngine.ReleaseLock(gilState);
+```
+
+A `using` statement may also be used to acquire and release the GIL:
+
+```csharp
+using (Py.GIL())
+{
+    PythonEngine.Exec("doStuff()");
+}
+```
 
 The AcquireLock and ReleaseLock methods are thin wrappers over the
 unmanaged `PyGILState_Ensure` and `PyGILState_Release` functions from
 the Python API, and the documentation for those APIs applies to
 the managed versions.
 
+## Passing C# Objects to the Python Engine
+
+This section demonstrates how to pass a C# object to the Python runtime.
+The example uses the following `Person` class:
+
+```csharp
+public class Person
+{
+    public Person(string firstName, string lastName)
+    {
+        FirstName = firstName;
+        LastName = lastName;
+    }
+
+    public string FirstName { get; set; }
+    public string LastName { get; set; }
+}
+```
+
+In order to pass a C# object to the Python runtime, it must be converted to a
+`PyObject`. This is done using the `ToPython` extension method. The `PyObject`
+may then be added to a dictionary of local variables and passed to the
+`PythonEngine.Exec` function:
+
+```csharp
+// create a person object
+Person person = new Person("John", "Smith");
+
+// acquire the GIL before using the Python interpreter
+using (Py.GIL())
+{
+    // convert the Person object to a PyObject
+    PyObject pyPerson = person.ToPython();
+
+    // create a Python variable "person"
+    PyDict locals = new PyDict();
+    locals["person"] = pyPerson;
+
+    // the person object may now be used in Python
+    string code = "fullName = person.FirstName + ' ' + person.LastName";
+    PythonEngine.Exec(code, null, locals.Handle);
+}
+```
+
 ## License
 
 Python for .NET is released under the open source MIT License.