Add proposals about two breaking changes before we cut Helmfile 1.0

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

docs/proposals/towards-1.0.md

@@ -0,0 +1,63 @@
+# Towards Helmfile 1.0
+
+I'd like to make two backward-incompatible changes to Helmfile and mark it as 1.0, so that we can better communicate with the current and future Helmfile users about our stance on maintaining Helmfile.
+
+## The changes in 1.0
+
+1. Forbid the use of `environments` and `releases` within a single helmfile.yaml part
+  - Helmfile currently relies on a hack called "double rendering" which no one understands correctly (I suppose) to solve the chicken-and-egg problem of rendering the helmfile template(which requires helmfile to parse `environments` as yaml first) and parsing the rendered helmfile as yaml(which requires helmfile to render the template first).
+  - By forcing (or print a big warning) the user to separate helmfile parts for `environments` and `releases`, it's very unlikely Helmfile needs double-rendering at all.
+  - After this change, every helmfile.yaml written this way:
+
+    ```
+    environments:
+      default:
+        values:
+        - foo: bar
+    releases:
+    - name: myapp
+      chart: charts/myapp
+      values:
+      - {{ .Values | toYaml | nindent 4 }}
+    ```
+    must be rewritten like:
+    ```
+    environments:
+      default:
+        values:
+        - foo: bar
+    ---
+    releases:
+    - name: myapp
+      chart: charts/myapp
+      values:
+      - {{ .Values | toYaml | nindent 4 }}
+    ```
+    It might not be a deal breaker as you already had to separate helmfile parts when you need to generate `environments` dynamically:
+    ```
+    environments:
+      default:
+        values:
+        - foo: bar
+    ---
+    environments:
+      default:
+        values:
+        - {{ .Values | toYaml | nindent 6}}}
+        - bar: baz
+    ---
+    releases:
+    - name: myapp
+      chart: charts/myapp
+      values:
+      - {{ .Values | toYaml | nindent 4 }}
+    ```
+2. Force `.gotmpl` (or `.tpl`) file extension for `helmfile.yaml` in case you want helmfile to render it as a go template before yaml parsing.
+  - As the primary maintainer of the project, I'm tired of explaining why Helmfile renders go template expressions embedded in a yaml comment. [The latest example of it](https://github.com/helmfile/helmfile/issues/127).
+  - When we first introduced helmfile the ability to render helmfile.yaml as a go template, I did propose it to require `.gotmpl` file extension to enable the feature. But none of active helmfile users and contributors at that time agreed with it (although I didn't have a very strong opinion on the matter either), so we enabled it without the extension. I consider it as a tech debt now.
+
+## After 1.0
+
+We won't add any backward-incompatible changes while in 1.x, as long as it's inevitable to fix unseen important bug(s).
+
+We also quit saying [Helmfile is in its early days](https://github.com/helmfile/helmfile#status) in your README as... it's just untrue today.