Add documentation for shared pointer class

docs/reference/api/platform/SharedPtr.md

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+## Shared Pointer
+
+A shared pointer is a "smart" pointer that retains ownership of an object by 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, as there are no weak pointers, no `make_shared`, no custom deleters, and so on.
+
+Usage: `SharedPtr<Class> ptr(new Class())`
+
+When `ptr` is passed around by a value, the copy constructor and destructor manage the reference count of the raw pointer. If the counter reaches zero, `delete` is called on the raw pointer.
+
+To avoid loops, use "weak" references by calling the original pointer directly with `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
+
+```
+#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
+}
+```