Enhance README with family filtering information

[lynxthecat]

Added family filtering section to README.md to provide guidance on blocking adult content.

[friendly-bits]

I think that some people may not like this use case [of enforcing content blocking]. Whether enforced content blocking is good or bad is a matter of opinion and I don't think we need to take a stand on this or suggest specific blocking practices. So I would rephrase the first inserted paragraph in a more neutral way:

Besides blocking ads, adblock-lean can also be leveraged to block unwanted content in general. For example, to block websites delivering malware or adult content.

And then shorten and slightly rephrase the second inserted paragraph:

Those interested in blocking adult content may want to consider setting the main DNS resolver to Cloudflare's [1.1.1.1 for Families](https://blog.cloudflare.com/introducing-1-1-1-1-for-families/), and then selectively blocking any further unwanted content with the help of adblock-lean.