Add information about getting Discord credentials

In the current Discord package docs, we don't provide information about obtaining the necessary credentials. This is the process:

1. In Discord, click on the channel name and select **Server settings**.
2. Click **Integrations** in the left nav.
3. Click **Webhooks**.
4. Create a new webhook or select an existing webhook.
5. Click **Copy Webhook URL**.
6. In a browser or from a command line, hit the webhook URL. It will return the webhook credentials.

##### Relevant URLs
- https://docs.influxdata.com/flux/v0.x/stdlib/contrib/chobbs/discord/
- https://docs.influxdata.com/flux/v0.x/stdlib/contrib/chobbs/discord/send/
- https://docs.influxdata.com/flux/v0.x/stdlib/contrib/chobbs/discord/endpoint/