Add adl_ranges (#413)

Co-authored-by: lsemprini <17140216+lsemprini@users.noreply.github.com>

doc/reference.md

@@ -512,6 +512,32 @@ constexpr bool enum_flags_contains(string_view value, BinaryPredicate p) noexcep
   magic_enum::enum_flags_test_any(Left|Down|Right, Down|Right); // -> "true"
   ```
 
+* Or, for enum types that are deeply nested in classes and/or namespaces, declare a function called `my_adl_info_struct adl_magic_enum_define_range(my_enum_type)` in the same namespace as `my_enum_type`, which magic_enum will find by ADL (because the function is in the same class/namespace as `my_enum_type`), and whose return type is a struct with `static constexpr` data members containing the same parameters as `magic_enum::customize::enum_range<my_enum_type>`
+  ```cpp
+  namespace Deeply::Nested::Namespace {
+  enum class my_enum_type { ... };
+  struct my_adl_info_struct {
+    static constexpr bool is_flags = true;
+    // you can also set min and max here (see Limitations document)
+    // static constexpr int min = ...;
+    // static constexpr int max = ...;
+  };
+  // - magic_enum will find this function by ADL
+  // - no need to ever define this function
+  my_adl_info_struct adl_magic_enum_define_range(my_enum_type); 
+  }
+  ```
+
+* As a shorthand, if you only want to set `is_flags` and not `min` or `max`, you can also use `magic_enum::customize::adl_info<is_flags_bool>` to avoid having to define `my_adl_info_struct` in your code:
+  ```cpp
+  namespace Deeply::Nested::Namespace {
+  enum class my_enum_type { ... };
+  // - magic_enum will find this function by ADL
+  // - no need to ever define this function
+  magic_enum::customize::adl_info<true> adl_magic_enum_define_range(my_enum_type); 
+  }
+  ```
+
 ## `is_unscoped_enum`
 
 ```cpp