Add init description to README

README.md

@@ -20,13 +20,38 @@ $ pip install tosfs
 * Init via `ak/sk`
 
 ```python
-
+from tosfs.core import TosFileSystem
+from tos import StaticCredentialsProvider
+
+tosfs = TosFileSystem(
+    key='ak',
+    secret='sk',
+    endpoint_url='http://tos-cn-beijing.volces.com',
+    region='cn-beijing',
+    credentials_provider=StaticCredentialsProvider, # optional
+)
 ```
 
 * Init via system env
-* 
-
-
 
+make sure these envs take effect:
 
+```shell
+export TOS_ACCESS_KEY=' your ak here '
+export TOS_SECRET_KEY=' your sk here '
+export TOS_ENDPOINT='http://tos-cn-beijing.volces.com'
+export TOS_REGION='cn-beijing'
+```
+then init `TosFileSystem` by setting `credentials_provider` to `EnvCredentialsProvider`
 
+```python
+import os
+from tosfs.core import TosFileSystem
+from tos import EnvCredentialsProvider
+
+tosfs = TosFileSystem(
+    endpoint_url=os.environ.get("TOS_ENDPOINT"),
+    region=os.environ.get("TOS_REGION"),
+    credentials_provider=EnvCredentialsProvider, # must
+)
+```