Add CloudFlare CDN notes in documentation

When using CloudFlare default settings, datapusher will start returning HTTP 403 Forbidden error.

This is because of the [Browser Integrity Check](https://support.cloudflare.com/hc/en-us/articles/200170086) feature of CloudFlare.

To correct, either turn off Browser Integrity Check for the whole CKAN instance or add a page rule for the pattern below with Browser Integrity Check off.

```
*.YOURCKANDOMAINHERE/en/dataset/* # e.g. *.ckan.org/en/dataset/*
```

This caused a lot of head-scratching on our end, before we finally pinned it down, so wanted to pass the info along.

No documentation file or test is named in the issue, so first locate the appropriate documentation area in the datapusher repository. Add the CloudFlare Browser Integrity Check explanation, remediation options, and example page-rule pattern; done means the guidance is easy to find and accurately reflects the reported HTTP 403 cause.

Written by the indexing model from the issue text.