Show how to use in template imports

[MichalBryxi]

• When I look at the index export I can see that for example is-empty is actually exported as export { default as isEmpty } from './helpers/is-empty.ts';
• But the README says that it's supposed to be used like: {{if (is-empty a)}}
• This together with the template imports example can lead people (it did to me) to conclusion that import should look like: import { is-empty } from 'ember-truth-helpers';
• This won't work.

So to avoid confusion I think the usage table in README should show all possible imports.