Create a new "Customizing Gitea" page under Advanced (#76) (#79)

* Create a new "Customizing Gitea" page under Advanced

For now, the only customization documented is the /robots.txt one.
Fixes #66

* Mention to restart Gitea when changing custom/robots.txt

Author: sztanpet

content/doc/advanced/customizing-gitea.en-us.md

@@ -0,0 +1,22 @@
+---
+date: "2017-01-20T16:00:00+02:00"
+title: "Customizing Gitea"
+slug: "customizing-gitea"
+weight: 9
+toc: false
+draft: false
+menu:
+  sidebar:
+    parent: "advanced"
+    name: "Customizing Gitea"
+    weight: 9
+    identifier: "customizing-gitea"
+---
+
+# Customizing Gitea
+
+The main way to customize Gitea is by using the `custom` folder. This is the central place to override and configure features.
+
+## Customizing /robots.txt
+
+To make Gitea serve your own `/robots.txt` (by default, an empty 404 status is served), simply create a file called `robots.txt` in the `custom` folder with the [expected contents](http://www.robotstxt.org/). **Note** that you have to restart Gitea for it to notice the new `robots.txt`.