DNS cache

[roccomuso]

I do understand that maybe it's not the purpose of this lib (feel free to close in case).
But would be useful adding a dns cache layer.
It's one of the common problem nowadays with huge number of requests. It doesn't cache dns lookups or anything. Maybe it's worth just mentioning this in Readme.

[nachogarcia]

We were having dns lookup issues in our server, greatly explained in https://medium.com/@amirilovic/how-to-fix-node-dns-issues-5d4ec2e12e95 and we didn't have access to setup dns cache on the nodes, so being able to enable caching in axios would have been great (even though I totally agree that this should be handled by Node or the OS).
Something like https://www.npmjs.com/package/axios-cached-dns-resolve

[chinesedfan]

Feel free to open a PR to update the document. And glad to see axios-cached-dns-resolve be added in ECOSYSTEM.md.