Design documentation updated

doc/nomnoml.md

@@ -0,0 +1,46 @@
+# nomnoml
+
+Graphs in the documentation are created with <http://www.nomnoml.com>.
+
+## Concepts
+
+```text
+[<abstract>Dimension|
+[base_dimension<Symbol, Unit>]<-[exp<Dimension, Num, Den>]
+[derived_dimension<Child, Unit, Exponent...>]<-[exp<Dimension, Num, Den>]
+[exp<Dimension, Num, Den>]<-[derived_dimension<Child, Unit, Exponent...>]
+]
+
+[<abstract>Quantity|
+[quantity<Dimension, Unit, Rep>]
+]
+
+[<abstract>Unit]<-[Dimension]
+[Dimension]<-[Quantity]
+[Unit]<-[Quantity]
+```
+
+## Units
+
+```text
+#direction: right
+
+[scaled_unit<Ratio, Unit>]<:-[unit<Child>]
+[scaled_unit<Ratio, Unit>]<:-[named_unit<Child, Symbol, PrefixType>]
+[scaled_unit<Ratio, Unit>]<:-[named_scaled_unit<Child, Symbol, PrefixType, Ratio, Unit>]
+[scaled_unit<Ratio, Unit>]<:-[prefixed_unit<Child, Prefix, Unit>]
+[scaled_unit<Ratio, Unit>]<:-[deduced_unit<Child, Dimension, Unit, Unit...>]
+```
+
+## Downcasting 1
+
+```text
+[detail::derived_dimension_base<exp<si::dim_length, 2>>]<:-[dim_area]
+```
+
+## Downcasting 2
+
+```text
+[downcast_base<detail::derived_dimension_base<exp<si::dim_length, 2>>>]<:-[detail::derived_dimension_base<exp<si::dim_length, 2>>]
+[detail::derived_dimension_base<exp<si::dim_length, 2>>]<:-[downcast_child<dim_area, detail::derived_dimension_base<exp<si::dim_length, 2>>>]
+[downcast_child<dim_area, detail::derived_dimension_base<exp<si::dim_length, 2>>>]<:-[dim_area]```