Add PingCAP to the user list (#959)

PingCAP is the team behind [TiDB](https://github.com/pingcap/tidb), which is using  Prometheus to monitor the database.

content/index.html

@@ -140,6 +140,7 @@ layout: jumbotron
           <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>
+          <a href="https://www.pingcap.com/"><img src="assets/company-logos/pingcap.png"/></a>
           <a href="https://www.qaware.de/"><img src="assets/company-logos/Qaware.png"/></a>
           <a href="http://www.quobyte.com/"><img src="assets/company-logos/quobyte.png"/></a>
           <a href="https://www.robustperception.io/"><img src="assets/company-logos/RobustPerception.png"/></a>