README.md: Add --enableGitInfo option

* Used in sitemap.xml <lastmod/>
* Used in ATOM feed <updated/>

README.md

@@ -13,13 +13,13 @@ $ git clone https://code.briarproject.org/briar/website.git
 Test page with local hugo webserver:
 
 ```
-hugo server -D
+hugo server -D --enableGitInfo
 ```
 
 Generate static hugo pages:
 
 ```
-hugo
+hugo --enableGitInfo
 ```
 
 ## Content Types