Add Related Packages section and list `curl_req`

[derekkraan]

Hi Woytek,

Following up on our discussion yesterday, here's a PR to start a list of related packages in the README of Req. I hope this helps people find interesting packages, and helps interesting packages be found.

I've moved the existing list of links down to this section.

Let me know if you'd like me to make any changes or have any other feedback.

Derek