Add instructions to run in Extension Development Host

INSTALL.md

@@ -1,10 +1,10 @@
-# Installation
+# Manual installation
 
 >This is a [Visual Studio Code](https://code.visualstudio.com/download) extension. It is best installed from the [on-line extensions marketplace](https://marketplace.visualstudio.com/items?itemName=svaberg.swmf-grammar), or from inside VS Code `Preferences/Extensions` and search for `swmf-grammar`.
 
 _Alternatively_, the extension can be built and installed manually as in the next section. This should only be required for developers.
 
-## Manual installation from source code
+## Build the extension
 
 1. Clone the repository and enter into the folder
 
@@ -19,14 +19,32 @@ _Alternatively_, the extension can be built and installed manually as in the nex
     npm install
     ```
 
-3. Create a package using `vsce`
+3. Compile with `npm`
+
+    ```bash
+    npm run compile
+    ```
+
+At this point the extension may be debugged in the Extension Development Host:
+
+```bash
+code .
+```
+
+followed by the `Debug/Start Debugging` dropdown (or the `F5` key). To observe the extension in action, view the file `demo.in`.
+
+To complete the manual installation, follow the remaining steps to manually create and install the package:
+
+## Package and install the extension
+
+4. Create a package using `vsce`
 
     ```bash
     vsce package
     ```
 
     This should generate a `swmf-grammar-?.?.?.vsix` file in the current directory.
-4. Manually install the extension in VS Code (you may want to replace the wildcards in the version number)
+5. Manually install the extension in VS Code (you may want to replace the wildcards in the version number)
 
     ```bash
     code --install-extension swmf-grammar-?.?.?.vsix