Provide public documented extension point to add resources

This follows the pattern we have in other packages.

Author: kzu

src/ThisAssembly.Resources/ThisAssembly.Resources.targets

@@ -28,9 +28,11 @@
     </EmbeddedResource>
   </ItemDefinitionGroup>
 
+  <Target Name="PrepareEmbeddedResources" DependsOnTargets="PrepareResourceNames" />
+
   <Target Name="_InjectResourcesAdditionalFiles"
           BeforeTargets="GenerateMSBuildEditorConfigFileShouldRun"
-          DependsOnTargets="PrepareResourceNames">
+          DependsOnTargets="PrepareEmbeddedResources">
     <ItemGroup>
       <EmbeddedResource Condition="!$([System.IO.Path]::IsPathRooted('%(RelativeDir)')) OR '%(Link)' != ''">
         <AreaPath Condition="!$([System.IO.Path]::IsPathRooted('%(RelativeDir)'))">%(RelativeDir)%(Filename)</AreaPath>

src/ThisAssembly.Resources/readme.md

@@ -52,6 +52,19 @@ The following MSBuild properties can be used to customize the generated code:
 | ThisAssemblyNamespace   | Sets the namespace of the generated `ThisAssembly` root class. If not set, it will be in the global namespace. |
 | ThisAssemblyVisibility  | Sets the visibility modifier of the generated `ThisAssembly` root class. If not set, it will be internal. |
 
+## Adding dynamic resources
+
+You can also provide additional embedded resources dynamically, by running a target before 
+`PrepareEmbeddedResources`:
+
+```xml
+  <Target Name="AddDynamicResources" BeforeTargets="PrepareEmbeddedResources">
+    <ItemGroup>
+      <EmbeddedResource Include="Content/Docs/$(Configuration).md" />
+    </ItemGroup>
+  </Target>
+```
+
 <!-- #resources -->
 <!-- include https://github.com/devlooped/sponsors/raw/main/footer.md -->
 <!-- exclude -->