Add 'usage' section with explanation of 'dump' command

content/doc/usage.en-us.md

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+---
+date: "2016-12-27T16:00:00+02:00"
+title: "Usage"
+slug: "usage"
+weight: 35
+toc: false
+draft: false
+menu:
+  sidebar:
+    name: "Usage"
+    weight: 35
+    identifier: "usage"
+---

content/doc/usage/backup-and-restore.en-us.md

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+---
+date: "2016-12-27T16:00:00+02:00"
+title: "Usage: Backup and Restore"
+slug: "backup-and-restore"
+weight: 11
+toc: true
+draft: false
+menu:
+  sidebar:
+    parent: "usage"
+    name: "Backup and Restore"
+    weight: 11
+    identifier: "backup-and-restore"
+---
+
+# Backup and Restore
+
+Gitea currently has a `dump` command that will save your installation to a zip
+file. There will be a `restore` command implemented at some point in the future.
+You will be able to use this to back up your installation, as well as make
+migrating servers easier.
+
+## Backup Command (`dump`)
+
+First, switch to the user running gitea: `su git` (or whatever user you are
+  using). Run `./gitea dump` in the gitea installation directory. You should
+  see some output similar to this:
+
+```
+2016/12/27 22:32:09 Creating tmp work dir: /tmp/gitea-dump-417443001
+2016/12/27 22:32:09 Dumping local repositories.../home/git/gitea-repositories
+2016/12/27 22:32:22 Dumping database...
+2016/12/27 22:32:22 Packing dump files...
+2016/12/27 22:32:34 Removing tmp work dir: /tmp/gitea-dump-417443001
+2016/12/27 22:32:34 Finish dumping in file gitea-dump-1482906742.zip
+```
+
+Inside the `gitea-dump-1482906742.zip` file, you will find the following:
+
+* `custom/conf/app.ini` - This is your server config.
+* `gitea-db.sql` - the mysql dump (TODO add info for other databases)
+* `gitea-repo.zip` - This zip will be a complete copy of your repo folder.
+   See Config -> repository -> `ROOT` for the location.
+* `log/` - this will contain various logs. You don't need these if you are doing
+   a migration.
+
+## Restore Command (`restore`)
+
+WIP: Does not exist yet.