Doc added how to setup email (#8520)

docs/content/doc/usage/email-setup.md

@@ -0,0 +1,33 @@
+---
+date: "2019-10-15T10:10:00+05:00"
+title: "Usage: Email setup"
+slug: "email-setup"
+weight: 12
+toc: true
+draft: false
+menu:
+  sidebar:
+    parent: "usage"
+    name: "Email setup"
+    weight: 12
+    identifier: "email-setup"
+---
+
+# Email setup
+
+- To use Gitea's built-in Email support, update the `app.ini` config file [mailer] section:
+
+```ini
+[mailer]
+ENABLED = true
+HOST    = mail.mydomain.com:587
+FROM    = gitea@mydomain.com
+USER    = gitea@mydomain.com
+PASSWD  = `password`
+```
+
+- Restart Gitea for the configuration changes to take effect.
+
+- To send a test email to validate the settings, go to Gitea > Site Administration > Configuration > SMTP Mailer Configuration.
+
+For the full list of options check the [Config Cheat Sheet]({{< relref "doc/advanced/config-cheat-sheet.en-us.md" >}})