Update readmeg

Author: Simon Brand

README.md

@@ -5,6 +5,18 @@ A lightweight non-owning reference to a callable.
 Clang + GCC: [![Linux Build Status](https://travis-ci.org/TartanLlama/function_ref.png?branch=master)](https://travis-ci.org/TartanLlama/function_ref)
 MSVC: [![Windows Build Status](https://ci.appveyor.com/api/projects/status/k5x00xa11y3s5wsg?svg=true)](https://ci.appveyor.com/project/TartanLlama/function_ref)
 
+Use `tl::function_ref` instead of `std::function` whenever you don't need to own the callable. The most common case for this is function parameters which aren't stored anywhere:
+
+```cpp
+void foo (function_ref<int(int)> func) {
+    std::cout << "Result is " << func(21); //42
+}
+
+foo([](int i) { return i*2; });
+```
+
+Full documentation available [here](https://function-ref.tartanllama.xyz/).
+
 ----------
 
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