diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 0000000..99396c0
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,37 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+`email-normalize` is a Python 3.11+ library that normalizes email addresses by stripping mailbox-provider-specific behaviors (plus addressing, period stripping, etc.). It uses async DNS (aiodns) to resolve MX records and match them against known providers, with a synchronous wrapper for non-async callers.
+
+## Commands
+
+```bash
+# Install dependencies (uses uv)
+uv sync --all-extras
+
+# Run tests
+uv run coverage run
+uv run coverage report
+
+# Run a single test
+uv run python -m unittest tests.test_normalize.MailboxProviderTestCase.test_google
+
+# Lint (ruff format + ruff check via pre-commit)
+uv run pre-commit run --all-files
+```
+
+## Architecture
+
+Two-module library under `email_normalize/`:
+
+- **`__init__.py`** — Core logic: `Normalizer` (async class with LFRU-cached MX lookups), `Result` dataclass, and `normalize()` sync wrapper. The `Normalizer` resolves MX records, matches them to providers, then applies provider-specific normalization rules. `skip_dns=True` mode bypasses MX lookups and uses a static `DomainMap` instead.
+
+- **`providers.py`** — Provider definitions: `Rules` flag enum (`PLUS_ADDRESSING`, `STRIP_PERIODS`, `LOCAL_PART_AS_HOSTNAME`), `MailboxProvider` base class, concrete provider classes (Apple, Fastmail, Google, etc.), `Providers` list (for MX matching), and `DomainMap` dict (for skip_dns mode).
+
+## Code Style
+
+- Ruff with 79-char line length, single quotes
+- See `pyproject.toml` `[tool.ruff.lint]` for the full rule selection
diff --git a/README.md b/README.md
index 3adfa09..88343be 100644
--- a/README.md
+++ b/README.md
@@ -1,35 +1,84 @@
 # email-normalize
 
-`email-normalize` is a Python 3.11+ library for returning a normalized email-address
-stripping mailbox-provider-specific behaviors such as "Plus addressing"
-(foo+bar@gmail.com).
+A Python 3.11+ library for normalizing email addresses by stripping
+mailbox-provider-specific behaviors such as plus addressing
+(`foo+bar@gmail.com`) and period ignoring (`f.o.o@gmail.com`).
 
 ![Version](https://img.shields.io/pypi/v/email-normalize.svg?)
 ![Status](https://github.com/gmr/email-normalize/workflows/Testing/badge.svg?)
 ![Coverage](https://img.shields.io/codecov/c/github/gmr/email-normalize.svg?)
 ![License](https://img.shields.io/pypi/l/email-normalize.svg?)
 
-## Example
+## Installation
+
+```bash
+pip install email-normalize
+```
+
+## Usage
+
+### Synchronous
+
+```python
+import email_normalize
+
+result = email_normalize.normalize('f.o.o+bar@gmail.com')
+print(result.normalized_address)  # foo@gmail.com
+print(result.mailbox_provider)    # Google
+print(result.mx_records)          # [(5, 'gmail-smtp-in.l.google.com'), ...]
+```
+
+### Async
+
+For use within an asyncio application, use the `Normalizer` class directly:
 
 ```python
+import asyncio
+
 import email_normalize
 
-# Returns ``foo@gmail.com``
-normalized = email_normalize.normalize('f.o.o+bar@gmail.com')
+
+async def main():
+    normalizer = email_normalize.Normalizer()
+    result = await normalizer.normalize('f.o.o+bar@gmail.com')
+    print(result.normalized_address)
+
+asyncio.run(main())
 ```
 
-## Currently Supported Mailbox Providers
+The `Normalizer` maintains a LFRU cache of MX lookups, making it efficient
+for batch processing.
+
+### Without DNS Lookups
+
+Use `skip_dns=True` to normalize against well-known domains without
+performing MX record lookups:
+
+```python
+result = email_normalize.normalize('user+tag@gmail.com', skip_dns=True)
+```
+
+This mode uses a static domain map and will not detect providers for
+custom domains.
+
+## Normalization Rules
+
+| Provider   | Plus Addressing | Strip Periods | Local Part as Hostname |
+|------------|:---------------:|:-------------:|:----------------------:|
+| Apple      | x               |               |                        |
+| Fastmail   | x               |               | x                      |
+| Google     | x               | x             |                        |
+| Microsoft  | x               |               |                        |
+| ProtonMail | x               |               |                        |
+| Rackspace  | x               |               |                        |
+| Yahoo      |                 |               |                        |
+| Yandex     | x               |               |                        |
+| Zoho       | x               |               |                        |
 
-- Apple
-- Fastmail
-- Google
-- Microsoft
-- ProtonMail
-- Rackspace
-- Yahoo
-- Yandex
-- Zoho
+- **Plus Addressing**: Strips everything after `+` in the local part
+- **Strip Periods**: Removes `.` from the local part
+- **Local Part as Hostname**: Extracts the subdomain as the local part (Fastmail custom domains)
 
-## Python Versions Supported
+## Documentation
 
-3.11+
+Full documentation is available at [gmr.github.io/email-normalize](https://gmr.github.io/email-normalize/).