Add mixin design doc again

Signed-off-by: Matej Gera <[email protected]>

mixin/README.md

@@ -4,9 +4,9 @@
 
 This directory contains extensible and customizable monitoring definitons for Thanos. [Grafana](http://grafana.com/) dashboards, and [Prometheus rules](https://prometheus.io/docs/prometheus/latest/configuration/recording_rules/) combined with documentation and scripts to provide easy monitoring experience for Thanos.
 
-The content of this project is written in [jsonnet](http://jsonnet.org/). This project could both be described as a package as well as a library.
+You can find more about monitoring-mixins in [the design document](https://github.com/monitoring-mixins/docs/blob/master/design.pdf), and you can check out other examples like [Prometheus Mixin](https://github.com/prometheus/prometheus/tree/master/documentation/prometheus-mixin).
 
-You can also check out other examples like [Prometheus Mixin](https://github.com/prometheus/prometheus/tree/master/documentation/prometheus-mixin).
+The content of this project is written in [jsonnet](http://jsonnet.org/). This project could both be described as a package as well as a library.
 
 ## Requirements