feat: Merge multiple docs inside a single helmfile.yaml (#270)

Resolves #254

docs/writing-helmfile.md

@@ -0,0 +1,72 @@
+# The Helmfile Best Practices Guide
+
+This guide covers the Helmfile’s considered patterns for writing advanced helmfiles. It focuses on how helmfile should be structured and executed.
+
+## Layering
+
+You may occasionally end up with many helmfiles that shares common parts like which repositories to use, and whichi release to be bundled by default.
+
+Use Layering to extract te common parts into a dedicated *library helmfile*s, so that each helmfile becomes DRY.
+
+Let's assume that your `helmfile.yaml` looks like:
+
+```
+{ readFile "commons.yaml" }}
+---
+{{ readFile "environments.yaml" }}
+---
+releases:
+- name: myapp
+  chart: mychart
+```
+
+Whereas `commons.yaml` contained a monitoring agent:
+
+```yaml
+releases:
+- name: metricbaet
+  chart: stable/metricbeat
+```
+
+And `environments.yaml` contained well-known environments:
+
+```yaml
+environments:
+  development:
+  production:
+```
+
+At run time, template expressions in your `helmfile.yaml` are executed:
+
+```yaml
+releases:
+- name: metricbaet
+  chart: stable/metricbeat
+---
+environments:
+  development:
+  production:
+---
+releases:
+- name: myapp
+  chart: mychart
+```
+
+Resulting YAML documents are merged in the order of occurrence,
+so that your `helmfile.yaml` becomes:
+
+```yaml
+environments:
+  development:
+  production:
+
+releases:
+- name: metricbaet
+  chart: stable/metricbeat
+- name: myapp
+  chart: mychart
+```
+
+Great!
+
+Now, repeat the above steps for each your `helmfile.yaml`, so that all your helmfiles becomes DRY.