fix(docs): add skeleton architecture doc

docs/arch.md

@@ -0,0 +1,69 @@
+# Architecture Guide
+
+This guide describes the program architecture.
+
+## Contents
+
+- [Architecture Guide](#architecture-guide)
+  - [Contents](#contents)
+  - [Purpose](#purpose)
+  - [Label](#label)
+    - [Flag Label](#flag-label)
+    - [State Label](#state-label)
+      - [State Label Value](#state-label-value)
+  - [Remote](#remote)
+    - [Github Remote](#github-remote)
+    - [Gitlab Remote](#gitlab-remote)
+  - [Sync](#sync)
+    - [Sync Issues](#sync-issues)
+    - [Sync Labels](#sync-labels)
+
+## Purpose
+
+`cautious-journey` exists to manage labels and their workflow.
+
+## Label
+
+While `cautious-journey` reads issue data, labels are what it cares about.
+
+### Flag Label
+
+Flag labels are set individually. While they may depend on or even remove each other,
+they are not connected directly.
+
+### State Label
+
+State labels are set from a group. Only one value from each state may be set at a time,
+and the highest priority value automatically replaces any others.
+
+#### State Label Value
+
+Individual values within the state.
+
+## Remote
+
+`cautious-journey` manipulates issues and labels that exist on some remote service.
+
+### Github Remote
+
+Connects to a Github repository.
+
+- `path`
+
+### Gitlab Remote
+
+Connects to a Gitlab repository.
+
+- `path`
+
+## Sync
+
+Update issue labels or the labels themselves.
+
+### Sync Issues
+
+Update issue labels.
+
+### Sync Labels
+
+Update project labels.