docs: define patterns vs conventions in README

Patterns are prescriptive — follow them.
Conventions are descriptive — study for ideas.

Clarifies repo purpose, directory structure, and how to
use patterns during development and review.

README.md

@@ -1,15 +1,31 @@
 # Elixir Patterns
 
-Idiomatic Elixir patterns extracted from the [Elixir source code](https://github.com/elixir-lang/elixir) with verified file:line citations.
+**Prescriptive.** Follow these when writing Elixir code.
+
+A pattern is a reusable solution to a recurring problem. Each one has:
+- **When to use** — the problem it solves
+- **When NOT to use** — where it causes harm
+- **Why** — the reasoning, not just the rule
+- **Source citations** — verified file:line from real codebases
+
+These are derived from what mature Elixir codebases *actually do*, not opinions or blog posts.
 
 ## Structure
 
-- `patterns/` — Core patterns (GenServer, error handling, data transforms, processes, testing, docs, typespecs, macros, behaviours, modules)
+- `patterns/` — what to do (behaviours, GenServer, error handling, testing, typespecs, etc.)
-- `smells/` — Anti-patterns and common mistakes the Elixir team avoids
+- `smells/` — what NOT to do (anti-patterns, common mistakes)
-- `changelog/` — Daily digest of merged Elixir PRs with discussion summaries
+- `sources/` — reference material from specific projects (Oban, elixir-lang). Study for ideas, don't copy blindly.
 
-## Philosophy
+## How to use
 
-These rules are derived from what the Elixir source code *actually does*, not opinions or blog posts. Every pattern cites specific files and line numbers.
+1. **Before writing code:** check if a relevant pattern exists
+2. **During review:** verify code follows documented patterns
+3. **If code deviates:** either fix it or document why the deviation is justified
 
-When unsure how to do something in Elixir, look at how Elixir core does it. This is how we define what "idiomatic" actually means.
+## Patterns vs Conventions
+
+**Pattern** = prescriptive. "When you face X, do Y." Language-scoped. Follow these.
+
+**Convention** = descriptive. "Project Z does it this way." Context-specific. Study for ideas — applying another project's conventions to yours without understanding their constraints causes harm.
+
+The `sources/` directory is convention material absorbed from thin repos. The `patterns/` directory is what you actually follow.