docs: Update GitInfo usage docs

docs/content/extras/gitinfo.md

@@ -18,6 +18,8 @@ Hugo provides a way to integrate Git data into your site.
 
 1. The Hugo site must be in a Git-enabled directory.
 1. The Git executable must be installed and in your system `PATH`.
+1. Enable the GitInfo feature in Hugo by using `--enableGitInfo` on the command
+   line or by setting `enableGitInfo` to `true` in your site configuration.
 
 ## The GitInfo Object
 

docs/content/overview/configuration.md

@@ -124,6 +124,8 @@ along with their current, default values:
     disableRSS:                 false
     # Do not build Sitemap file
     disableSitemap:             false
+    # Enable GitInfo feature
+    enableGitInfo:              false
     # Build robots.txt file
     enableRobotsTXT:            false
     # Do not render 404 page