Merge pull request #46 from ericsebesta/patch-2

Update README.md

Author: akrzemi1

README.md

@@ -29,7 +29,7 @@ Clang 3.2, Clang 3.4, G++ 4.7.2, G++ 4.8.1. Tested only with libstdc++, versions
 Known Issues
 ------------
 
- - Currently, the reference (and the only known) impementation of certain pieces of functionality explore what C++11 identifies as undefined behavior (see national body comment FI 15: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3770.html#FI15). This is mostly why Optional was removed from C++14 and put into Library Fundamentals TS. Luckily what the Standard identifies as UB is well defined on all known platforms. We expect that the C++14 wil fix this problem, so that our trick becomes well-defined.
+ - Currently, the reference (and the only known) implementation of certain pieces of functionality explore what C++11 identifies as undefined behavior (see national body comment FI 15: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3770.html#FI15). This is mostly why Optional was removed from C++14 and put into Library Fundamentals TS. Luckily what the Standard identifies as UB is well defined on all known platforms. We expect that the C++14 wil fix this problem, so that our trick becomes well-defined.
  - In libstdc++ versions below 20130531 the constructor taking `initializer_list` argument is not `constexpr`. This is because `initializer_list` operations are not `constexpr` in C++11. This works however in version 20130531. It is also not enabled for libc++ because I do not have access to it and do nto know if it provides `constexpr` `initializer_list`.
  - In G++ 4.7.2 and 4.8.0 member function `value_or` does not have rvalue reference overload. These compilers do not support rvalue overloding on `*this`.
  - In order for the library to work with slightly older compilers, we emulate some missing type traits. On some platforms we cannot correctly detect the available features, and attempts at workarounds for missing type trait definitions can cause compile-time errors. Define macro `TR2_OPTIONAL_DISABLE_EMULATION_OF_TYPE_TRAITS` if you know that all the necessary type traits are defined, and no emulation is required.