Add Kumina's company logo to the list of users. (#755)

Kumina a company based in the Netherlands doing managed operations for
Linux-based systems. Kumina has switched over to Prometheus over the
last year and is now using it as its exclusive monitoring system for
some of its customers.

In the process, Kumina has developed (and is still developing) metrics
exporters for many pieces of commonly used Open Source software. Most of
these can be found on our company's GitHub page:

	https://github.com/kumina

Kumina is promoting the use of Prometheus and affiliated projects, like
Kubernetes, by giving talks about them at Open Source conferences in the
Netherlands.

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@@ -134,6 +134,7 @@ layout: jumbotron
           <a href="http://improbable.io/"><img src="assets/company-logos/Improbable.png"/></a>
           <a href="https://www.jodel-app.com/"><img src="assets/company-logos/Jodel.png"/></a>
           <a href="https://www.justwatch.com/"><img src="assets/company-logos/JustWatch.png"/></a>
+          <a href="https://www.kumina.nl/"><img src="assets/company-logos/Kumina.png"/></a>
           <a href="http://www.mavensecurities.com/"><img src="assets/company-logos/MavenSecurities.png"/></a>
           <a href="http://www.outbrain.com/"><img src="assets/company-logos/Outbrain.png"/></a>
           <a href="https://www.percona.com/"><img src="assets/company-logos/Percona.png"/></a>