Add doc for infix

doc/gendoc

@@ -14,6 +14,7 @@ extract fuse
 extract identity
 extract implicit
 extract indirect
+extract infix
 extract invoke
 extract is_callable
 extract lambda

doc/src/infix.md

@@ -0,0 +1,40 @@
+infix
+=====
+
+Description
+-----------
+
+The `infix` function adaptor allows the function to be used as an infix
+operator. The operator must be placed inside the angle brackets(ie `<`
+and `>`).
+
+Synopsis
+--------
+
+    template<class F>
+    constexpr infix_adaptor<F> infix(F f);
+
+Requirements
+------------
+
+F must be:
+
+    BinaryFunctionObject
+    MoveConstructible
+
+Example
+-------
+
+    struct plus_f
+    {
+        template<class T, class U>
+        T operator()(T x, U y) const
+        {
+            return x+y;
+        }
+    };
+    
+    constexpr infix_adaptor<plus_f> plus = {};
+    int r = 3 <plus> 2;
+    assert(r, 5);
+