Update example in README

[NiedziolkaMichal]

Following example doesn't work, because addValue has protected modifier:

policy.sandbox().get().addValue("allow-something-new");

Instead of removing it, it was replaced with usage of SandboxDirective.Value enum, that I've added in #258:

policy.sandbox().get().setValue(SandboxDirective.Value.AllowPopupsToEscapeSandbox, false);

[kingthorin]

Seems reasonable to me, assuming 258 is merged.