entry.fileComment is documented wrong

[thejoshwolfe]

In the README, the field is called entry.comment. In the code, it's called entry.fileComment. It's possible that users have written code that relies on either on of these, so we should make them both exist as aliases of one another.

The README will now document that entry.fileComment is the recommended way, and entry.comment is deprecated.

We should also add a test for file comments. If it's in the README, it needs first-class support, and that means tests.