show latest version prominently on website

[TomRoche]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2019.04.30
• I've searched the bugtracker for similar feature requests including closed ones

Description

I recently had a problem with your usually-excellent product, which told me

ERROR: oZmmPVg-yz0: "token" parameter not in video info for unknown reason; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see  https://yt-dl.org/update  on how to update.

I checked the latter link, but am using Debian, hence that was useless. Eventually I just reinstalled, but, IMHO, It Would Be Useful to

1. show the current-release-version stamp (currently=2019.04.30) prominently on (e.g.)

• the GitHub "homepage", e.g. @ https://github.com/ytdl-org/youtube-dl#installation
• the update page referenced in the stock error message
• the latest-version item on the CONTRIBUTING page

2. update the version stamp in those docs when building new releases

This should also be easy (depending on your build process), since you seem to be updating the new-issue template with the current-version-stamp already.

[dstftw]

This will barely have any effect. There are clear instructions and current version in the new issue template - nobody reads it.