docs: add integration docs (#1277)

* add intergration docs

* add link to intergrations.md from the main README.md
* start integrations.md doc

* requested changes

* no .md

* fix links and add notes about opentsdb aggregation

README.md

@@ -36,6 +36,7 @@ Concretely the aims of the project are:
 * [Introduction blog post](https://improbable.io/games/blog/thanos-prometheus-at-scale)
 * [Benchmarks](https://github.com/improbable-eng/thanos/tree/master/benchmark)
 * [Proposals](docs/proposals)
+* [Integrations](docs/integrations.md)
 
 ## Features
 

docs/integrations.md

@@ -0,0 +1,21 @@
+---
+title: Integrations
+type: docs
+menu: thanos
+slug: /integrations.md
+---
+
+# Integrations 
+
+## StoreAPI 
+
+[StoreAPI](https://github.com/improbable-eng/thanos/blob/master/pkg/store/storepb/rpc.proto) is a common proto interface for gRPC component that can connect to [Querier](components/query.md) in order to fetch the metric series. Natively Thanos implements [Sidecar](components/sidecar.md) (local Prometheus data), [Ruler](components/rule.md) and [Store gateway](components/store.md). This solves fetching series from Prometheus or Prometheus TSDB format, however same interface can be used to fetch metrics from other storages. 
+
+Below you can find some public integrations with other systems through StoreAPI:
+
+### OpenTSDB
+
+[Geras](https://github.com/G-Research/geras) is an OpenTSDB integration service which can connect your OpenTSDB cluster to Thanos. Geras exposes the Thanos Storage  API, thus other Thanos components can query OpenTSDB via Geras, providing a unified  query interface over OpenTSDB and Prometheus.
+
+Although OpenTSDB is able to aggregte the data, it's not supported by Geras at the moment. 
+