docs/CaseStudies.md: add case study for zhihu (#956)

docs/CaseStudies.md

@@ -6,6 +6,7 @@ and feel free asking for references, reviews and additional case studies from re
 See also [articles about VictoriaMetrics from our users](https://github.com/VictoriaMetrics/VictoriaMetrics/wiki/Articles#third-party-articles-and-slides).
 
 
+* [zhihu](#zhihu)
 * [adidas](#adidas)
 * [CERN](#cern)
 * [COLOPL](#colopl)
@@ -19,6 +20,20 @@ See also [articles about VictoriaMetrics from our users](https://github.com/Vict
 * [ARNES](#arnes)
 * [Idealo.de](#idealode)
 
+## zhihu
+
+[zhihu](https://www.zhihu.com) is the largest chinese question-and-answer website. We use VictoriaMetrics to store and use Graphite metrics, and we shared the [promate](https://github.com/zhihu/promate) solution in our [单机 20 亿指标，知乎 Graphite 极致优化！](https://qcon.infoq.cn/2020/shenzhen/presentation/2881)([slides](https://static001.geekbang.org/con/76/pdf/828698018/file/%E5%8D%95%E6%9C%BA%2020%20%E4%BA%BF%E6%8C%87%E6%A0%87%EF%BC%8C%E7%9F%A5%E4%B9%8E%20Graphite%20%E6%9E%81%E8%87%B4%E4%BC%98%E5%8C%96%EF%BC%81-%E7%86%8A%E8%B1%B9.pdf)) talk at [QCon 2020](https://qcon.infoq.cn/2020/shenzhen/).
+
+Numbers:
+
+- Active time series: ~2500 Million
+- Datapoints: ~20 Trillion
+- Ingestion rate: ~1800k/s
+- Disk usage: ~20 TiB
+- Index size: ~600 GiB
+- The average query rate is ~3k per second (mostly alert queries).
+- Query duration: median is ~40ms, 99th percentile is ~100ms.
+
 
 ## adidas