Document what is synced and in which directions

Reading https://lwc-gus-bot.herokuapp.com my initial takeaway was that Git2Gus:
- Creates new GUS work items in response to the relevant labels being applied to a GitHub issue
- Allows manually linking an existing work item to a GitHub issue by using the `@...@` notation in the issue description
- Would automatically mark GUS work items as INTEGRATE (or whatever is specified for `statusWhenClosed`) when the GitHub issue was closed

However it wasn't clear:
- whether any of these would be synced in the other direction (for example does closing the GUS work item close the GH issue?)
- whether linking an existing work item would cause any changes to the work item or github issue at time of linking?
- whether a linked work item (either one automatically created, or one manually linked) would have the work item type or priority synced from GitHub to GUS as further changes happened in the future?
- what other fields may or may not be synced (such as title, description, comments etc)?

From reading the source it would seem that...

1. On the most part Git2Gus is one-way sync from GitHub to GUS (with a couple of exceptions, such as GitHub labels being updated on issues at time of manual linking). I suppose this makes sense, given the project's name!
2. All linked items (whether auto-created or manual linked) have their record type and priority synced from GitHub to GUS on an ongoing basis. (The only exception being that the GUS priority is never lowered by Git2Gus, only raised)
3. Automatically created work items also have their titles and descriptions updated any time the GitHub issue changes. However manually linked work items do not.
4. Closing a work item does not currently close the GitHub issue (#15)
5. Metadata such as assignee, comments, etc are not set/synced

I think it would be really useful to document the above, so that its clearer what Git2Gus currently does :-)