Update README.md - Adding Source Maps Section

[hbobenicio]

Just adding a Source Maps section to the README.md docs.

This is the result on the issue #53.

After some painful debugging section, I just realized that optimize-css-assets-webpack-plugin would pass cssProcessorOptions to cssnano, and the defaults used by this plugin would result in no source-maps.

This doc section can save some time for others that are using the default configuration and want to use source-maps, but don't know how it is truncated.

Anyway... it's open to discussion and improvements. Here it is.

[hbobenicio]

@NMFR there it is

[hbobenicio]

I'm closing this PR because maybe this is not actually the correct way to configure it.
It works for me, but it may confuse more people about the actual recommended way to achieve this (source maps + bundle otimization).
We're still discussing it at issue #53 .