Update the contribution guide to include steps for running tests

[lawynnj]

The contributing guide (https://project-books.github.io/docs/contributing/) is missing steps on how to execute tests.

This would be helpful to contributors who are unfamiliar with the environment.

[knjk04]

Thanks for creating this issue. I've transferred it over to this docs repository.

We may need separate instructions for running tests using Gradle (Books API) and Maven (Book Project). I think it would be good if we keep the contributing guide as short as possible, but perhaps we can link to a new 'How to' page that describes the steps. What do you think?

[lawynnj]

That sounds like a good idea. Makes sense to have separate instructions for the two projects. Do we intend on storing documentation for both projects in this docs repository long term?

[knjk04]

Initially, we created this repository to hold common documentation. We could maintain separate wikis, but one advantage of having separate documentation in this docs repository is contributors can look in one place for everything. What do you think?