readme breakpoints that match bootstrap 4

[tarekadam]

Bro! Nicely done! This is a fantastic module! Just wanted to add a bit of readme that would have been good when I got started. Thanks Again!!

[ysle]

so based on the relevant bootsrap docs, it's like :

import VueMq from 'vue-mq'
Vue.use(VueMq, {
	breakpoints: {
		xs: 576,
		sm: 768,
		md: 992,
		lg: 1200,
		xl: Infinity
	}
})

nicely matching with a template of :

<span class="d-inline d-sm-none">XS</span>
<span class="d-none d-sm-inline d-md-none">SM</span>
<span class="d-none d-md-inline d-lg-none">MD</span>
<span class="d-none d-lg-inline d-xl-none">LG</span>
<span class="d-none d-xl-inline">XL</span>