add optional dependencies (#100)

README.md

@@ -24,6 +24,29 @@ database migrations.
 
 **ORM is still under development: We recommend pinning any dependencies with `orm~=0.1`**
 
+---
+
+## Installation
+
+```shell
+$ pip install orm
+```
+
+You can install the required database drivers with:
+
+```shell
+$ pip install orm[postgresql]
+$ pip install orm[mysql]
+$ pip install orm[sqlite]
+```
+
+Driver support is provided using one of [asyncpg][asyncpg], [aiomysql][aiomysql], or [aiosqlite][aiosqlite].
+Note that if you are using any synchronous SQLAlchemy functions such as `engine.create_all()` or [alembic][alembic] migrations then you still have to install a synchronous DB driver: [psycopg2][psycopg2] for PostgreSQL and [pymysql][pymysql] for MySQL.
+
+---
+
+## Quickstart
+
 **Note**: Use `ipython` to try this from the console, since it supports `await`.
 
 ```python
@@ -179,6 +202,13 @@ See TypeSystem for [type-specific validation keyword arguments][typesystem-field
 * `orm.JSON()`
 
 [sqlalchemy-core]: https://docs.sqlalchemy.org/en/latest/core/
+[alembic]: https://alembic.sqlalchemy.org/en/latest/
+[psycopg2]: https://www.psycopg.org/
+[pymysql]: https://github.com/PyMySQL/PyMySQL
+[asyncpg]: https://github.com/MagicStack/asyncpg
+[aiomysql]: https://github.com/aio-libs/aiomysql
+[aiosqlite]: https://github.com/jreese/aiosqlite
+
 [databases]: https://github.com/encode/databases
 [typesystem]: https://github.com/encode/typesystem
 [typesystem-fields]: https://www.encode.io/typesystem/fields/

setup.py

@@ -51,6 +51,12 @@ def get_packages(package):
     package_data={PACKAGE: ["py.typed"]},
     data_files=[("", ["LICENSE.md"])],
     install_requires=["databases>=0.2.1", "typesystem"],
+    extras_require={
+        "postgresql": ["asyncpg"],
+        "mysql": ["aiomysql"],
+        "sqlite": ["aiosqlite"],
+        "postgresql+aiopg": ["aiopg"]
+    },
     classifiers=[
         "Development Status :: 3 - Alpha",
         "Environment :: Web Environment",