Instance defaults example in README

[esimons]

Section/Content To Improve
https://github.com/axios/axios/blob/master/README.md#custom-instance-defaults

Suggested Improvement
Remove example code which is known to be problematic in the version of Axios which is currently tagged as latest (0.18.0):

- // Alter defaults after instance has been created
- instance.defaults.headers.common['Authorization'] = AUTH_TOKEN;

Relates to #1395
It would be great to replace this block with a note on how to safely set default headers on a created instance of Axios without inadvertently modifying global defaults, but at a minimum the docs shouldn't be directing users towards trouble with its code examples.

Relevant File(s):
README.md

[LazaroOnline]

I'm also suffering this exact issue, in my case adding security Auth token to the headers is adding that to all the instances. Resulting in a security risk if I didn't notice, and making the other instances fail and send tokens where they are not supposed to be sent.
Please fix this and the related issue!

[freakymittal]

I couldn't find a way to do thing without modifying the globals. Either we can add a transform request intercepter there or there has to be another function provided in the axios instance which can do the same.

[chinesedfan]

Closed by #3539.