Added usage example.

Author: petrbroz

README.md

@@ -1,3 +1,21 @@
 # autodesk-forge-sdk
 
-Unofficial [Autodesk Forge](https://forge.autodesk.com) SDK for Python 3.6 and above.
+Unofficial [Autodesk Forge](https://forge.autodesk.com) SDK for Python 3.* and above.
+
+## Usage
+
+```bash
+pip3 install autodesk-forge-sdk
+```
+
+```python
+import os
+from autodesk_forge_sdk import AuthenticationClient
+
+FORGE_CLIENT_ID = os.environ["FORGE_CLIENT_ID"]
+FORGE_CLIENT_SECRET = os.environ["FORGE_CLIENT_SECRET"]
+
+client = AuthenticationClient()
+auth = client.authenticate(FORGE_CLIENT_ID, FORGE_CLIENT_SECRET, ["viewables:read"])
+print(auth["access_token"])
+```