Add build info + Jenkins badge to README

[heathermiller]

Build for this repo is now here:
https://scala-webapps.epfl.ch/jenkins/job/build_docs_scala_website/

[SethTisue]

curious why this change was made?

(and does it make sense that we're still using the github-pages gem?)

[SethTisue]

ah, over at #803 I see:

Heads up though; @fsalvi just moved this build to EPFL machines, which means we can now add plugins and use https (plugins are required for #806)
The build is here: https://scala-webapps.epfl.ch/jenkins/job/build_docs_scala_website/

[heathermiller]

Yes, merging the look and feel of the new redesign of scala-lang required plugins which were impossible to use on GitHub Pages.

That, and https wasn't possible on GitHub pages with a custom domain.

[Philippus]

I assume we would need something like https://wiki.jenkins.io/display/JENKINS/Embeddable+Build+Status+Plugin installed on the Jenkins machine

[SethTisue]

the README has a "backend" section with a link to the Jenkins info. tbh, I'm tempted to just close this as not being worth the effort, really