Update mixin readme (prometheus-community#1022)

Author: cristiangreco

mixin/README.md

@@ -2,8 +2,23 @@
 
 This is a Prometheus [Monitoring Mixin](https://monitoring.mixins.dev/) that comes with pre-defined dashboards.
 
-It can be installed e.g. with [Grizzly](https://grafana.github.io/grizzly):
+It can be installed e.g. with [Grizzly](https://grafana.github.io/grizzly).
+
+First, install [jsonnet-bundler](https://github.com/jsonnet-bundler/jsonnet-bundler) with
+
+```
+go install -a github.com/jsonnet-bundler/jsonnet-bundler/cmd/jb@latest
+```
+
+Then install all the dependencies of this mixin:
+
+```
+jb install
+```
+
+Finally, install `Grizzly` and apply the mixin to your Grafana instance:
 
 ```
-$ grr apply mixin.libsonnet 
+go install github.com/grafana/grizzly/cmd/grr@latest
+grr apply mixin.libsonnet
 ```