Improvement to way we suggest environment variables

Docs currently suggest that best practice is to set the env variables `CLOUDFLARE_ACCOUNT_ID`, `CLOUDFLARE_AUTH_EMAIL`, `CLOUDFLARE_AUTH_KEY`, `CLOUDFLARE_ZONE_ID` in their environment variables every time they run a script

The best practice is to maintain a set of config files `config/dev.yml` that contains stage-specific config and then read these vars into the `serverless.yml`
e.g.

```
custom:
deployVars: ${file(./config/config.${opt:stage}.yml)}

provider:
name: cloudflare
config:
accountId: ${self:custom.deployVars.CLOUDFLARE_ACCOUNT_ID}
zoneId: ${self:custom.deployVars.CLOUDFLARE_ZONE_ID}
```