Add explanation about C++14 [[deprecated]] standard attribute (#49)

* Add [[deprecated]] attribute to C++14

CPP14.md

@@ -11,6 +11,7 @@ C++14 includes the following new language features:
 - [decltype(auto)](#decltypeauto)
 - [relaxing constraints on constexpr functions](#relaxing-constraints-on-constexpr-functions)
 - [variable templates](#variable-templates)
+- [\[\[deprecated\]\] attribute](#deprecated-attribute)
 
 C++14 includes the following new library features:
 - [user-defined literals for standard library types](#user-defined-literals-for-standard-library-types)
@@ -146,6 +147,15 @@ template<class T>
 constexpr T e  = T(2.7182818284590452353);
 ```
 
+### [[deprecated]] attribute
+C++14 introduces the `[[deprecated]]` attribute to indicate that a unit (function, class, etc) is discouraged and likely yield compilation warnings. If a reason is provided, it will be included in the warnings.
+```c++
+[[deprecated]]
+void old_method();
+[[deprecated("Use new_method instead")]]
+void legacy_method();
+```
+
 ## C++14 Library Features
 
 ### User-defined literals for standard library types