Add a section in the README about withCustomDio

[brunoocasali]

After the #266 implementation, it is now possible to use a custom implementation of the HTTP library in this package.

This issue aims to create a small section in the README explaining how to use this feature. Check the test file test/custom_dio_test.dart for more information.

[ahmednfwela]

do we need to update the code samples as well ?

[brunoocasali]

Not for this case, this is only a dart thing, the code-samples is only for features shared between all the integrations :)