Update Qiskit app documentation

Proposed changes

Briefly describe the changes you've made here. Remember to add a link to the preview page of your branch.

[x] I have followed the instructions in the Contributing and Styleguide documents.
[x] My pull request passes the automated tests. (If not, visit pull requests at Travis CI and have a look at the job log.)
[x] I have included a link to the appropriate preview page (select your branch from the list).