docs: add bin/renderlocal to README w/examples

- examples should be copy/paste-able from the root of the repo
- included:
  - single and multiple `--env` arguments
  - with and without `--out` option

README.md

@@ -608,3 +608,60 @@ before enabling impersonation, any user on the cluster that is allowed to create
 razeedeploy resources will be able to impersonate any other user. Once the ImpersonationWebhook
 controller is installed and all necessary config and authorizations in place, impersonation
 can be safely enabled in the `razeedeploy-config` configmap.
+
+## Local Testing with `bin/renderlocal`
+
+`bin/renderlocal` can be used to see what the resources generated by a
+MustacheTemplate would look like.
+
+`bin/renderlocal` arguments:
+
+- `--mtp` a path to the template you want to render.
+- `--env` a path to a file containing `ConfigMaps`, `Secrets`, or other
+  resources used to render the template. Any number of `--env` files can be
+  added; at least one is required.
+- `--out` if provided, the path to write the resulting rendered resources
+  [OPTIONAL]
+
+### Example
+
+```shell
+bin/renderlocal --mtp test/example/hello-mtp.yaml --env test/example/hello-cm.yaml
+
+{..."failed to get env: {\"name\":\"greeting\",... # expected warning
+kind: ConfigMap
+apiVersion: v1
+metadata:
+  name: result
+  namespace: default
+data:
+  result: hello, Razee!
+```
+
+```shell
+bin/renderlocal --mtp test/example/hello-mtp.yaml \
+--env test/example/hello-cm.yaml --env test/example/alternate-greeting-cm.yaml
+
+kind: ConfigMap
+apiVersion: v1
+metadata:
+  name: result
+  namespace: default
+data:
+  result: howdy, Razee!
+```
+
+```shell
+bin/renderlocal --mtp test/example/hello-mtp.yaml \
+--env test/example/hello-cm.yaml --env test/example/alternate-greeting-cm.yaml \
+--out /tmp/output.yaml \
+&& cat /tmp/output.yaml
+
+kind: ConfigMap
+apiVersion: v1
+metadata:
+  name: result
+  namespace: default
+data:
+  result: howdy, Razee!
+```

test/example/alternate-greeting-cm.yaml

@@ -0,0 +1,8 @@
+---
+kind: ConfigMap
+apiVersion: v1
+metadata:
+  name: alternate-greeting-cm
+  namespace: default
+data:
+  greeting: howdy

test/example/hello-cm.yaml

@@ -0,0 +1,8 @@
+---
+kind: ConfigMap
+apiVersion: v1
+metadata:
+  name: hello-cm
+  namespace: default
+data:
+  name: Razee

test/example/hello-mtp.yaml

@@ -0,0 +1,28 @@
+---
+kind: MustacheTemplate
+apiVersion: "deploy.razee.io/v1alpha2"
+metadata:
+  name: hello-mtp
+  namespace: default
+spec:
+  env:
+  - name: name
+    valueFrom:
+      configMapKeyRef:
+        name: hello-cm
+        key: name
+  - name: greeting
+    valueFrom:
+      configMapKeyRef:
+        name: alternate-greeting-cm
+        key: greeting
+    optional: true
+    default: "hello"
+  templates:
+  - kind: ConfigMap
+    apiVersion: v1
+    metadata:
+      name: result
+      namespace: default
+    data:
+      result: "{{greeting}}, {{name}}!"