Update README.md

Author: k0kubun

README.md

@@ -8,6 +8,66 @@ Ruby interface to call [sqldef](https://github.com/k0kubun/sqldef).
 gem 'sqldef'
 ```
 
+## Usage
+### Download
+
+You can download `mysqldef`, `psqldef`, or `sqlite3def`.
+
+```rb
+Sqldef.bin = './bin'
+Sqldef.download(:psqldef)
+```
+
+`download` is automatically executed by following methods too.
+
+### Export
+
+You can export the database schema to a file.
+
+```rb
+Sqldef.export(
+  command:  :psqldef,
+  host:     host,
+  port:     port,
+  user:     user,
+  password: password,
+  database: database,
+  path:     'db/schema.sql',
+)
+```
+
+### Dry Run
+
+You can show DDLs to be executed.
+
+```rb
+Sqldef.dry_run(
+  command:  :psqldef,
+  host:     host,
+  port:     port,
+  user:     user,
+  password: password,
+  database: database,
+  path:     'db/schema.sql',
+)
+```
+
+### Apply
+
+You can run DDLs to match the schema.
+
+```rb
+Sqldef.apply(
+  command:  :psqldef,
+  host:     host,
+  port:     port,
+  user:     user,
+  password: password,
+  database: database,
+  path:     'db/schema.sql',
+)
+```
+
 ## License
 
 The gem is available as open source under the terms of the [MIT License](https://opensource.org/licenses/MIT).