Add link to good first issues and maintenance repo to "Getting Involved"

[leouieda]

Under the "Join the development" section, we instruct people on the types of work they can do to get involved in the project. We link to our contributing guide and the fatiando/contributing repo. But it can be hard to search all projects for "good first issues" or other tasks. This sections should include:

• A link to http://good-first-issues.fatiando.org which leads to a search for issues across all our repositories
• A link to the https://github.com/fatiando/maintenance repository

These should include some text on what these things are about, not just the links.