Git - clarify git extension API usage (microsoft#192139)

* Git - clarify git extension API usage

* Fixed white space

Author: lszomoru

extensions/git/README.md

@@ -14,7 +14,13 @@ The Git extension exposes an API, reachable by any other extension.
 2. Include `git.d.ts` in your extension's compilation.
 3. Get a hold of the API with the following snippet:
 
- ```ts
- const gitExtension = vscode.extensions.getExtension<GitExtension>('vscode.git').exports;
- const git = gitExtension.getAPI(1);
- ```
+	```ts
+	const gitExtension = vscode.extensions.getExtension<GitExtension>('vscode.git').exports;
+	const git = gitExtension.getAPI(1);
+	```
+	**Note:** To ensure that the `vscode.git` extension is activated before your extension, add `extensionDependencies` ([docs](https://code.visualstudio.com/api/references/extension-manifest)) into the `package.json` of your extension:
+	```json
+	"extensionDependencies": [
+		"vscode.git"
+	]
+	```