Issue microsoft#701 updated readme.md for webapi to make it clearer h…

…ow one can include plugins in build output for publishing (microsoft#752)

### Motivation and Context

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  1. Why is this change required? Issue microsoft#701 
2. What problem does it solve? Not clear in readme that this step is
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  3. What scenario does it contribute to?
  4. If it fixes an open issue, please link to the issue here. microsoft#701 
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### Description

Readme updated with example of what is required to include prompt.txt in
build output

webapi/README.md

@@ -301,6 +301,14 @@ Alternatively,
     </Content>
    ```
 
+   You will also need to include a line for each skprompt.txt file you have for you plugins to ensure they are included in the build output.
+
+   ```xml
+   <None Update="Plugins\SemanticPlugins\{PluginName}\skprompt.txt">
+     <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+   </None>
+   ```
+
 3. Change the respective directory option to use an absolute path or a different base path, but make sure that the files are accessible from that location.
 
 ### Add Custom Setup to Chat Copilot's Kernel