Update README.md

Author: angularsen

README.md

@@ -30,6 +30,7 @@ See [Upgrading from 3.x to 4.x](https://github.com/angularsen/UnitsNet/wiki/Upgr
 * [Operator overloads](#operator-overloads) for arithmetic on quantities
 * [Parse and ToString()](#culture) supports cultures and localization
 * [Dynamically parse and convert](#dynamic-parsing) quantities and units
+* [Custom units](#custom-units)
 * [Example: Creating a unit converter app](#example-app)
 * [Example: WPF app using IValueConverter to parse quantities from input](#example-wpf-app-using-ivalueconverter-to-parse-quantities-from-input)
 * [Precision and accuracy](#precision)
@@ -225,6 +226,36 @@ UnitConverter.ConvertByName(1, "Length", "Centimeter", "Millimeter"); // 10 mm
 UnitConverter.ConvertByAbbreviation(1, "Length", "cm", "mm"); // 10 mm
 ```
 
+### <a name="custom-units"></a>Custom units
+
+Units.NET allows you to add your own units and quantities at runtime, to represent as `IQuantity` and reusing Units.NET for parsing and converting between units.
+
+Read more at [Extending-with-Custom-Units](https://github.com/angularsen/UnitsNet/wiki/Extending-with-Custom-Units).
+
+#### Map between unit enum values and unit abbreviations
+```c#
+// Map unit enum values to unit abbreviations
+UnitAbbreviationsCache.Default.MapUnitToDefaultAbbreviation(HowMuchUnit.Some, "sm");
+UnitAbbreviationsCache.Default.GetDefaultAbbreviation(HowMuchUnit.Some); // "sm"
+UnitParser.Default.Parse<HowMuchUnit>("sm");  // HowMuchUnit.Some
+
+var unitConverter = UnitConverter.Default;
+unitConverter.SetConversionFunction<HowMuch>(HowMuchUnit.Lots, HowMuchUnit.Some, x => new HowMuch(x.Value * 2, HowMuchUnit.Some));
+unitConverter.SetConversionFunction<HowMuch>(HowMuchUnit.Tons, HowMuchUnit.Lots, x => new HowMuch(x.Value * 10, HowMuchUnit.Lots));
+unitConverter.SetConversionFunction<HowMuch>(HowMuchUnit.Tons, HowMuchUnit.Some, x => new HowMuch(x.Value * 20, HowMuchUnit.Some));
+```
+
+#### Convert between units of custom quantity
+```c#
+var from = new HowMuch(10, HowMuchUnit.Tons);
+IQuantity Convert(HowMuchUnit toUnit) => unitConverter.GetConversionFunction<HowMuch>(from.Unit, toUnit)(from);
+
+Console.WriteLine($"Convert 10 tons to:");
+Console.WriteLine(Convert(HowMuchUnit.Some)); // 200 sm
+Console.WriteLine(Convert(HowMuchUnit.Lots)); // 100 lts
+Console.WriteLine(Convert(HowMuchUnit.Tons)); // 10 tns
+```
+
 ### <a name="example-app"></a>Example: Creating a dynamic unit converter app
 [Source code](https://github.com/angularsen/UnitsNet/tree/master/Samples/UnitConverter.Wpf) for `Samples/UnitConverter.Wpf`<br/>
 [Download](https://github.com/angularsen/UnitsNet/releases/tag/UnitConverterWpf%2F2018-11-09) (release 2018-11-09 for Windows)