[Contributing] Update Alpha doc (#4898)
[Contributing] Update Alpha doc (#4898)

Update alpha_components.md so that future contributors don't get stuck on imports. The import statements can't follow the standard convention if they are in the MaterialComponentsAlpha phase then a component will need to import `MaterialComponentsAlpha` instead of `MaterialComponents`.
material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios