README.adoc: add a link to Download Swarm Client in the top bar

[jimklimov]

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your master branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

This is a small change aiming for https://plugins.jenkins.io/swarm/ page to display the download link visibly (there is one, lost in the wall of text, which makes quick new deployments bumpy). Sadly, the repo does not have a "latest" symlink for this like the jenkins.war distributions, so the best we can point to is the location with all versions (alternately have to update the README with every release to bump the link).

[jimklimov]

Seems unit tests (not relevant for the docs change) could not connect to localhost:8080 on the Windows builder.
This did not happen in the other docs PRs I've opened today, though.

[basil]

When previewing this at https://github.com/jimklimov/swarm-plugin/tree/doc-downloadlink, the badge looks like there is an em or en dash between "swarm" and "client", while looking at the badge at this URL it appears properly as a hyphen. I think there must be something going on with AsciiDoc escaping. Try playing around with this to see if you can get it to show up properly as a hyphen in the badge on the AsciiDoc page.

[jimklimov]

Hopefully fixed now, added a bit of "white lie" to puzzle asciidoc for long enough :)