A little doc for tracker config

[timbl]

No description provided.

[brownhoward]

When attempting to setup an issue tracker using the example turtle files, an error is displayed when attempting to render the issue tracker:

Error: This tracker has no issueClass
  undefined
    Error: This tracker has no issueClass  undefined
        at Object.render (https://khoward.inrupt.net/mashlib.min.js:21:108263)  undefined
        at _.propertyTable (https://khoward.inrupt.net/mashlib.min.js:21:35517)  undefined
        at u (https://khoward.inrupt.net/mashlib.min.js:21:42368)  undefined
        at https://khoward.inrupt.net/mashlib.min.js:21:43979  undefined
        at https://khoward.inrupt.net/mashlib.min.js:33:106895

Perhaps replace example index.ttl with https://khoward.inrupt.net/public/FooTracker/index.ttl which includes issueClass, propertyList, and issueCategory configurations?

The https://github.com/solid/issue-pane/blob/doc/Documentation/Configuration.md is also missing a section on issueClass.