Add README file to loki-mixin (#6083)

* Add README file to loki-mixin

* Apply suggestions from code review

Co-authored-by: Kaviraj Kanagaraj <kavirajkanagaraj@gmail.com>

* Address comments

* Add JSONNET_PATH variable

Co-authored-by: Kaviraj Kanagaraj <kavirajkanagaraj@gmail.com>

production/loki-mixin/README.md

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+# loki-mixin
+
+loki-mixin is a jsonnet library containing a set of Loki monitoring dashboards, alerts and rules collected based on our experience operating Loki in Grafana Cloud.
+
+## Dashboards
+
+To test the dashboards against a local grafana & Loki setup perform the following steps.
+
+### Pre-requisites
+
+* jb is a jsonnet package manager
+To install it follow the instructions at: https://github.com/jsonnet-bundler/jsonnet-bundler.
+
+* Grizzly is a tool for managing jsonnet dashboards in Grafana: https://github.com/grafana/grizzly.
+To install it follow the instructions at: https://grafana.github.io/grizzly/installation/.
+
+* Make sure you have the latest dependencies in the `vendor` directory by running the following command in `production/loki-mixin`:
+
+```shell
+jb install
+```
+
+* On your Grafana instance create an API key with role 'Admin' under Configuration > API keys. 
+Copy this key for the next step.
+
+### Testing dashboards
+
+To test the dashboard in your local grafana instance, in directory `production/loki-mixin` run the command:
+
+```shell
+GRAFANA_URL=http://localhost:3000 GRAFANA_TOKEN=<API_KEY> JSONNET_PATH=$(pwd)/lib:$(pwd)/vendor grr watch ./ dashboards.libsonnet
+```
+
+`grr watch` will detect changes when you save files and try to add/update dashboards:
+
+```shell
+INFO[0000] Watching for changes
+INFO[0005] Changes detected. Applyingdashboards.libsonnet
+INFO[0007] Applying 10 resources
+Dashboard.reads-resources added
+Dashboard.writes-resources added
+Dashboard.reads updated
+Dashboard.writes updated
+...
+```
+
+**Disclaimer:** Since these dashboards are used on our own production setup, these contain very specific configurations to our cloud environment which may need to overridden for other setups and use-cases.