Add clearer documentation for rule.exclude

[eliperelman]

In neutrinojs/neutrino#1150 there was confusion about the API of rule.exclude.

We should probably add clearer documentation about it to the README here :-)

(Edited by @edmorley to make it clearer that this is a documentation issue not a bug)

[homeant]

use console.log(JSON.stringify(config.toConfig())); Cannot print reg

[edmorley]

Hi @homeant, this issue is about adding documentation for the rule.exclude feature, whereas that seems unrelated? Generally it's best to keep issues to just one topic. It sounds like you're after toString() - see: https://github.com/neutrinojs/webpack-chain#inspecting-generated-configuration