Add documentation for shared pointer class

Author: Donatien Garnier

docs/reference/api/platform/SharedPtr.md

@@ -0,0 +1,41 @@
+## Shared Pointer
+
+A shared pointer is a "smart" pointer that retains ownership of an object using
+reference counting accross all smart pointers referencing that object.
+
+It is similar to the `std::shared_ptr` class introduced in C++11,
+however this is not a compatible implementation (no weak pointer, no make_shared, no custom deleters, etc.)
+
+Usage: `SharedPtr<Class> ptr(new Class())`
+
+When `ptr` is passed around by value the copy constructor and
+destructor manages the reference count of the raw pointer.
+If the counter reaches zero, `delete` is called on the raw pointer.
+
+To avoid loops, "weak" references should be used by calling the original
+pointer directly through `ptr.get()`.
+
+
+### SharedPtr class reference
+
+[![View code](https://www.mbed.com/embed/?type=library)](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/class_shared_ptr.html)
+
+### Shared pointer example
+
+```cpp
+#include "platform/SharedPtr.h"
+  
+void test() {
+    struct MyStruct { int a; };
+
+    // Create shared pointer
+    SharedPtr<MyStruct> ptr( new MyStruct );
+
+    // Increase reference count
+    SharedPtr<MyStruct> ptr2( ptr );
+
+    ptr = NULL; // Reference to the struct instance is still held by ptr2
+
+    ptr2 = NULL; // The raw pointer is freed
+}
+```