* Use `yarn` instead of `npm` in docs and scripts

Signed-off-by: Andrew Ferrazzutti <andrewf@element.io>

* Enforce node and yarn versions

Signed-off-by: Andrew Ferrazzutti <andrewf@element.io>

* Give up on trying to auto-enforce yarn 1.x

Instead, just document how to install it

Signed-off-by: Andrew Ferrazzutti <andrewf@element.io>

* Changelog: Clarify that Yarn is required

Signed-off-by: Andrew Ferrazzutti <andrewf@element.io>

Add a docker badge :)

Signed-off-by: pacien <pacien.trangirard@pacien.net>

Signed-off-by: pacien <pacien.trangirard@pacien.net>

Following the link for creating a new application actually results in

> Application not found!
> There was no application found with ID create.

This error page has no clear way to actually create an application. Going up to https://discordapp.com/developers/applications/, there's at least a button in the top-right for "New Application".

Things need to be stored in a volume, so we also add a bunch of configuration options for moving the persistence around.

Helps to differentiate the start command and the command that generates
the config. I believe `npm start` also makes sure the code is up to
date.

* Check WebHooks
* Fix spacing of checkboxes so they render correctly on GitHub

This should fix #2, untested since I'm on my phone.

Add doc explaining how puppeting works