Add docs for codespaces.openFiles (devcontainers#105)

Author: joshaber

supporting.md

@@ -97,19 +97,34 @@ If your Codespaces project needs additional permissions for other repositories,
 
 ```json
 "customizations": {
-		// Configure properties specific to Codespaces.
-		"codespaces": {
-			"repositories": {
-				"my_org/my_repo": {
-					"permissions": {
-						"issues": "write"
-					}
+	// Configure properties specific to Codespaces.
+	"codespaces": {
+		"repositories": {
+			"my_org/my_repo": {
+				"permissions": {
+					"issues": "write"
 				}
 			}
 		}
+	}
 }
 ```
 
+You can customize which files are initially opened when the codespace is created:
+```json
+"customizations": {
+	// Configure properties specific to Codespaces.
+	"codespaces": {
+		"openFiles": [
+			"README"
+			"src/index.js"
+		]
+	}
+}
+```
+
+The paths are relative to the root of the repository. They will be opened in order, with the first file activated.
+
 Note that currently Codespaces reads these properties from devcontainer.json, not image metadata.
 
 #### <a href="#codespaces-specific-limitations" name="codespaces-specific-limitations" class="anchor"> Product specific limitations </a>