feat: update readme.

README.md

@@ -5,3 +5,45 @@
 </p>
 
 Ursa is a UI library for building cross-platform UIs with Avalonia UI.
+
+## How to use
+1. Ursa
+
+Add nuget package:
+```bash
+dotnet add package Irihi.Ursa --version 0.1.0-beta20230702
+```
+
+You can now use Ursa controls in your Avalonia Application.
+```xaml
+<Window
+    ...
+    xmlns:u="https://irihi.tech/ursa"
+    ...>
+    <StackPanel Margin="20">
+        <u:ButtonGroup Classes="Solid Warning">
+            <Button Content="Hello" />
+            <Button Content="World" />
+        </u:ButtonGroup>
+        <u:TagInput />
+    </StackPanel>
+</Window>
+```
+
+2. Ursa.Themes.Semi
+
+To make Ursa controls show up in your application, you need to reference to a theme package designed for Ursa. 
+Ursa.Themes.Semi is a theme package for Ursa inspired by Semi Design. You can add it to your project by following steps.
+
+Add nuget package:
+```bash
+dotnet add package Irihi.Ursa.Themes.Semi --version 0.1.0-beta20230702
+```
+
+Include Styles in application:
+```xaml
+    <Application.Styles>
+        <StyleInclude Source="avares://Semi.Avalonia/Themes/Index.axaml" />
+        <StyleInclude Source="avares://Ursa.Themes.Semi/Index.axaml" />
+    </Application.Styles>
+```