Consider moving mixin location

[bcardarella]

The README states you should use:

import RouterScrollMixin from 'ember-router-scroll';

However the mixin is actually located elsewhere, so you have to do:

import RouterScrollMixin from 'ember-router-scroll/mixins/router-scroll';

which is OK but the line in the README seems better. To achieve this you can move ember-router-scroll/addon/mixins/router-scroll.js to ember-router-scroll/addon/index.js

[briangonzalez]

@bcardarella Agreed.

@bennycwong Mind taking a stab?