Add PingCAP to the user list (prometheus#959)

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

Author: QueenyJin

content/index.html

@@ -140,6 +140,7 @@ <h1><i class="fa fa-github"></i> Open Source</h1>
           <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>