Add migration guide (#396)

* Add migration guide

* Update Migration-guide.md based on PR feedback

Running-Mastodon/Migration-guide.md

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+Migration guide
+====
+
+Sometimes, for various reasons, you may want to migrate your Mastodon instance from one server to another. Fortunately this is not too difficult of a process, although it may result in some downtime.
+
+**Note:** this guide was written with Ubuntu Server 16.04 in mind; your mileage may vary for other setups.
+
+Basic steps
+----
+
+1. Set up a new Mastodon server using the [Production Guide](https://github.com/tootsuite/documentation/blob/master/Running-Mastodon/Production-guide.md) (however, don't run `db:setup`).
+2. Stop Mastodon on the old server (e.g. `systemctl stop 'mastodon-*.service'`).
+3. Dump and load the Postgres database using the instructions below.
+4. Copy the `system/` files using the instructions below.
+5. Run `RAILS_ENV=production bundle exec rails mastodon:feeds:build` to rebuild the home timelines for each user.
+6. Start Mastodon on the new server.
+7. Update your DNS settings to point to the new server.
+8. Update or copy your Nginx configuration, re-run LetsEncrypt as necessary.
+9. Enjoy your new server!
+
+Detailed steps
+----
+
+### What data needs to be migrated
+
+At a high level, you'll need to copy over the following:
+
+- The `~/live/public/system` directory, which contains user-uploaded images and videos
+- The Postgres database (using [pg\_dump](https://www.postgresql.org/docs/9.1/static/backup-dump.html))
+- The `~/live/.env.production` file, which contains server config and secrets
+
+Less crucially, you'll proably also want to copy the following for convenience:
+
+- The nginx config (under `/etc/nginx/sites-available/default`)
+- The systemd config files (`/etc/systemd/system/mastodon-*.service`), which may contain your server tweaks and customizations
+- The pgbouncer configuration under `/etc/pgbouncer` (if you're using it)
+
+
+
+### Dump and load Postgres
+
+Instead of running `db:setup`, we're going to create an empty Postgres database using the `template0` database (which is useful when restoring a Postgres dump, [as described in the pg\_dump documentation](https://www.postgresql.org/docs/9.1/static/backup-dump.html#BACKUP-DUMP-RESTORE)).
+
+Run this as the `mastodon` user on your new system:
+
+```bash
+createdb -T template0 mastodon_production
+```
+
+Next, create a Postgres dump on the old system (again, as the `mastodon` user):
+
+```bash
+pg_dump mastodon_production > dump.sql
+```
+
+Copy the `dump.sql` file over, using `scp` or `rsync`. Then on the new system, run:
+
+```bash
+psql mastodon_production < dump.sql
+```
+
+This will copy the Postgres database from the old system to the new one. You may see a warning about `no privileges could be revoked for "public"`, but [you can ignore it](https://confluence.atlassian.com/bamkb/errors-or-warnings-appear-when-importing-postgres-database-dump-829036698.html).
+
+Note that you will need to re-run this process from scratch (i.e. on a fresh database) if the old database changes after the dump.
+
+### Copy `system/` files
+
+This will probably take some time, and you'll want to avoid re-copying unnecessarily, so using `rsync` is recommended. On your old machine, as the `mastodon` user, run:
+
+```bash
+rsync -avz ~/live/public/system/ mastodon@example.com:~/live/public/system/
+```
+
+You'll want to re-run this if any of the files on the old server change.
+
+You can also copy over any other important files, such as `.env.production` and the nginx, systemd, and pgbouncer config files.
+
+### During migration
+
+You can edit the `~/live/public/500.html` page on the old machine if you want to show a nice error message to let existing users know that a migration is in progress.
+
+You'll probably also want to set the DNS TTL to something small (30-60 minutes) about a day in advance, so that DNS can propagate quickly once you point it to the new IP address.
+
+### After migrating
+
+You can check [whatsmydns.net](http://whatsmydns.net/) to see the progress of DNS propagation. To jumpstart the process, you can always edit your own `/etc/hosts` file to point to your new server so you can start playing around with it early.