Add Backups Guide (#414)

Maintaining-Mastodon/Backups-Guide.md

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+# Mastodon Backups Guide
+
+A production Mastodon instance has several pieces of data that needs to be
+regularly backed up to protect against data loss.
+
+Data that needs to be backed up regularly:
+* PostgreSQL database
+* User generated content (images, avatars, headers)
+
+Data that needs to be backed up at least once:
+* Mastodon application secrets (see
+  [Production Guide](../Running-Mastodon/Production-Guide.md) for more details)
+
+In the following sub-sections, some suggestions on how to backup all of this
+data will be provided.
+
+## PostgreSQL database
+
+Mastodon uses a PostgreSQL database as it's relational database. This database
+needs to be backed up regularly.
+
+We will be providing two methods that can be used to backup your Mastodon
+PostgreSQL database.
+
+### wal-e
+
+[wal-e](https://github.com/wal-e/wal-e) is a program that is, "designed to
+perform continuous archiving of PostgreSQL WAL files and base backups."
+
+wal-e allows you to store the backups on services such as [AWS S3](https://aws.amazon.com/s3/) or it's work-alikes.
+
+The documentation provided by wal-e is excellent, so it is recommended to read
+[it](https://github.com/wal-e/wal-e/blob/master/README.rst) for help with
+configuring wal-e.
+
+#### wal-e configuration example
+
+One example of a wal-e backup schedule and retention is to do daily base backups
+with retention of the last seven base backups into a AWS S3 bucket.
+
+What this example looks like as a cron job:
+
+```
+@daily envdir /etc/wal-e.d/env /usr/local/bin/wal-e backup-push /var/lib/postgresql/9.5/
+@weekly envdir /etc/wal-e.d/env /usr/local/bin/wal-e delete --confirm retain 7
+```
+
+### pg_dump
+
+[pg_dump](https://www.postgresql.org/docs/9.5/static/app-pgdump.html) is an utility that comes with PostgreSQL that can be used extract the contents of a
+PostgreSQL database.
+
+Please read the documentation page linked above for all relevant usage details.
+
+Here is an example:
+```sh
+# Dump Mastodon's PostgreSQL database into a SQL-script file
+pg_dump mastodon_production > mastodon_production.sql
+```
+
+If you are looking for a pre-written set of scripts to maintain PostgreSQL backups using pg_dump, you can find them [here](https://wiki.postgresql.org/wiki/Automated_Backup_on_Linux).
+
+## User generated content (images, avatars, headers)
+
+Day to day usage of a production Mastodon instance will result in user
+generated content such as images, user avatars and headers. This data needs
+to be backed up regularly.
+
+If the [Production Guide](../Running-Mastodon/Production-Guide.md) is used, this
+content will be stored in `/home/mastodon/live/public/system`.
+
+Various methods can be used to back up this directory, here are some:
+* [rsync](https://www.digitalocean.com/community/tutorials/how-to-use-rsync-to-sync-local-and-remote-directories-on-a-vps) to an offsite backup server
+* Synced to an AWS S3 bucket using the [AWS CLI tool](http://docs.aws.amazon.com/cli/latest/reference/s3/sync.html)
+
+## Mastodon application secrets
+
+It is also highly recommended to keep your application configuration file
+.env.production backed up.
+
+If you the [Production Guide](../Running-Mastodon/Production-Guide.md) is used
+this file is stored in `/home/mastodon/live/.env.production`.
+
+That file contains application secrets used for things like 2 factor authentication and VAPID keys used for Web Push notifications.