feat: Add a make rule to generate helm docs

It's going to be easier for contributors, if they don't have to figure
out the way to update README.md. So, I guess, it makes sense to have a
Makefile with a corresponding rule.

helm/.gitignore

@@ -0,0 +1 @@
+bin

helm/Makefile

@@ -0,0 +1,8 @@
+LOCALBIN ?= $(shell pwd)/bin
+$(LOCALBIN):
+	mkdir -p $(LOCALBIN)
+
+.PHONY: gen_docs
+gen_docs: ## Generate helm documentation
+	test -s $(LOCALBIN)/setup-envtest || GOBIN=$(LOCALBIN) go install github.com/norwoodj/helm-docs/cmd/helm-docs@latest
+	./bin/helm-docs --template-files=./README.md.gotmpl --sort-values-order file

helm/README.md

@@ -69,3 +69,7 @@ helm install sql_exporter/sql-exporter
 To configure `target`, `jobs`, `collector_files` please refer to the [documentation](https://github.com/burningalchemist/sql_exporter/blob/master/documentation/sql_exporter.yml) in the source repository. These values are not set by default.
 
 It's also possible to define collectors (i.e. metrics and queries) in separate files, and specify the filenames in the `collector_files` list. For that we can use `CollectorFiles` field (check `values.yaml` for the available example).
+
+## Dev Notes
+
+After updating default `Values`, please execute `make gen_docs` to update the `README.md` file

helm/README.md.gotmpl

@@ -60,3 +60,7 @@ helm install sql_exporter/sql-exporter
 To configure `target`, `jobs`, `collector_files` please refer to the [documentation](https://github.com/burningalchemist/sql_exporter/blob/master/documentation/sql_exporter.yml) in the source repository. These values are not set by default.
 
 It's also possible to define collectors (i.e. metrics and queries) in separate files, and specify the filenames in the `collector_files` list. For that we can use `CollectorFiles` field (check `values.yaml` for the available example).
+
+## Dev Notes
+
+After changing default `Values`, please execute `make gen_docs` to update the `README.md` file. Readme file is generated by the `helm-docs` tool, so make sure not to edit it manually.