Add note about not placing files inside /var/www/html

[myzhang1029]

Existing files inside /var/www/html get deleted by the upgrade script. This update documents that. (#1165)

[J0WI]

Can you just refer to upgrade.exclude?

[myzhang1029]

Yeah, I agree. That would probably be better.

[myzhang1029]

Suggested change

	Please note that anything inside `/var/www/html` will be removed by the installation script, except for `config`, `data`, `custom_apps`, `themes` and `version.php`. So make sure you do not leave other important files inside.
	Please note that anything inside `/var/www/html` will be removed by the installation script, except for those listed in [upgrade.exclude](https://github.com/nextcloud/docker/blob/master/upgrade.exclude). So make sure you do not leave other important files inside.

[bsiddans]

I'd love to help get this one over the line - in addition to the issue mentioned above there's also #1528. This looks like a straight-forward solution compared to #1548.

With regard to the proposed addition, would a slight tweak to the wording help make clear that this issue is relevant for both installation, and upgrades? E.g.:

Users should note that data inside the main folder (/var/www/html) may be removed during installation and upgrades. Users should consider:

• Mounting storage volumes to locations outside of /var/www/html; and/or
• Listing files and folders that should not be removed from the main folder in upgrade.exclude.

I'm not sure what the status of this PR is but I'm happy to submit a new one if required.

[J0WI]

I think you need to start from scratch with this PR. It's too outdated.

[J0WI]

Fixed in #1862