First draft of docs for weak callbacks

doc/weak.md

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+#Weak Callbacks
+
+## NanWeakCallbackInfo
+Argument to weak callbacks.
+You may need to free any external resources attached to the object.
+```c++
+template<typename T>
+class NanWeakCallbackInfo {
+ public:
+  typedef void (*Callback)(const NanWeakCallbackInfo<T>& data);
+
+  v8::Isolate *GetIsolate() const;
+
+   /**
+    *   Get the parameter that was associated with the weak handle.
+    */
+  T *GetParameter() const;
+
+   /**
+    *   Get pointer from internal field, index can be 0 or 1.
+    */
+  void *GetInternalField(int index) const;
+};
+```
+
+### Example
+```c++
+void weakCallback(const NanWeakCallbackInfo<int> &data) {
+  int *parameter = data.GetParameter();
+  delete parameter;
+}
+
+NanPersistent<v8::Object> obj;
+int *data = new int(0);
+obj.SetWeak(data, callback, NanWeakCallbackType::kParameter);
+```