Include comment with latest generation time/generation about in addon repository.

*I'm submitting a ...*
- [ ] bug report
- [X] feature request
- [ ] support request

## General infomration

### Prerequisites

* [X] Are you running the latest version?

### Description
I like to get information about the generation of the repository list, so I can rule out caching (Cloudflare or local) problems.
This would allow someone to determine the reason for discrepancies to the release list in github.
Related #551

### Steps to Reproduce

Take a look at https://johtru.com/repo/repo.php
and https://github.com/asciidisco/plugin.video.netflix/releases
They differ currently.

**Expected behavior:** Some metadata (as xml comment) in the response alongside current repo information. Like: When was this generated.

**Actual behavior:** the contents of the repo are shown.

### Installation

* [ ] I installed the plugin via zip from the Releases page
* [X] I´m using the Netflix Repo
* [ ] I´m using a different source (Please tell which)

Start by inspecting the repository-generation endpoint represented by https://johtru.com/repo/repo.php and compare its response with the GitHub release list. Done means the XML response includes a comment or equivalent metadata showing when the repository list was generated, so caching discrepancies can be investigated.

Written by the indexing model from the issue text.