Add README.md

Signed-off-by: beorn7 <beorn@grafana.com>

docs/node-mixin/README.md

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+# Node Mixin
+
+_This is work in progress. We aim for it to become a good role model for alerts
+and dashboards eventually, but it is not quite there yet._
+
+The Node Mixin is a set of configurable, reusable, and extensible alerts and
+dashboards based on the metrics exported by the Node Exporter. The mixin create
+recording and alerting rules for Prometheus and suitable dashboard descriptions
+for Grafana.
+
+To use them, you need to have `jsonnet` (v0.13+) and `jb` installed. If you
+have a working Go development environment, it's easiest to run the following:
+```bash
+$ go get github.com/google/go-jsonnet/cmd/jsonnet
+$ go get github.com/jsonnet-bundler/jsonnet-bundler/cmd/jb
+```
+
+_Note: The make targets `lint` and `fmt` need the `jsonnetfmt` binary, which is
+currently not included in the Go implementation of `jsonnet`. For the time
+being, you have to install the [C++ version of
+jsonnetfmt](https://github.com/google/jsonnet) if you want to use `make lint`
+or `make fmt`._
+
+Next, install the dependencies by running the following command in this
+directory:
+```bash
+$ jb install
+```
+
+You can then build the Prometheus rules files `node_alerts.yaml` and
+`node_rules.yaml`:
+```bash
+$ make node_alerts.yaml node_rules.yaml
+```
+
+You can also build a directory `dashboard_out` with the JSON dashboard files
+for Grafana:
+```bash
+$ make dashboards_out
+```
+
+For more advanced uses of mixins, see
+https://github.com/monitoring-mixins/docs.
+