Add if/else BreakpointContainer render pattern to README examples

[saxoncameron]

Something like this:

<BreakpointContainer>
    {bpName => resolveBp(bpName, 's') ? <p>One</p> : <p>two</p>}
</BreakpointContainer>

Is superior to:

<Breakpoint query="0,s">
    <p>One</p>
</Breakpoint>
<Breakpoint query="s">
    <p>Two</p>
</Breakpoint>

But not intuitive for first-time users. Should find an appropriate place for this in the README amongst the other patterns. Maybe the way the patterns are currently presented is a little verbose.