Extend README

README.md

@@ -88,6 +88,117 @@ package manager.
 
 ## Usage
 
+Flask-EasyJWT is used exactly as [EasyJWT](https://github.com/BMeu/EasyJWT). Therefore, this section only describes the
+specific features of Flask-EasyJWT and the basic usage. For detailed explanations on how to use EasyJWT (for example,
+optional claims, registered claims such as `aud`, `iat`, and `sub`, or verifying third-party tokens), see
+[its documentation](https://easyjwt.readthedocs.org/en/latest/#usage).
+
+### Application Setup
+
+You do not need to initialize Flask-EasyJWT with your Flask application. All you have to do (although even this is,
+strictly speaking, not required), is to specify some default settings for all of your tokens in the configuration of
+your Flask application. These settings are:
+
+
+| Configuration Key        | Description |
+|--------------------------|-------------|
+| `EASYJWT_KEY`            | The key that will be used for encoding and decoding all tokens. If `EASYJWT_KEY` is not specified, Flask-EasyJWT will fall back to Flask's `SECRET_KEY` configuration value. |
+| `EASYJWT_TOKEN_VALIDITY` | The validity of each token after its creation. This value can be given as a string (that is parsable to an integer), an integer, or a `timedelta` object. The former two are interpreted in seconds. |
+
+You can specify these configuration values as any other configuration values in your Flask application, for example,
+using a mapping in your code:
+
+```python
+from datetime import timedelta
+from flask import Flask
+
+app = Flask(__name__)
+app.config.update(
+    EASYJWT_KEY='Super secret key',
+    EASYJWT_TOKEN_VALIDITY=timedelta(minutes=7)
+)
+```
+
+In this example, all tokens will (by default) be encoded using the (not so secure) string `Super secret key` and will
+be valid for seven minutes after they have been created (i.e., after the `create()` method has been called on the token
+object).
+
+Of course, any other way of specifying the configuration values will work as well (see
+[Flask's documentation](https://flask.palletsprojects.com/en/1.1.x/config/)).
+
+### Token Specification & Usage
+
+Tokens are specified and used exactly as with [EasyJWT](https://easyjwt.readthedocs.org/en/latest/#usage):
+
+```python
+from flask_easyjwt import FlaskEasyJWT
+
+# Define the claims of your token.
+class MySuperSimpleJWT(FlaskEasyJWT):
+
+    def __init__(self, key):
+        super().__init__(key)
+
+        # Define a claim `name`.
+        self.name = None
+
+# Assuming we are within a Flask app context. 
+
+# Create a token with some values.
+token_object = MySuperSimpleJWT()
+token_object.name = 'Zaphod Beeblebrox'
+token = token_object.create()
+
+# Verify the created token.
+verified_token_object = MySuperSimpleJWT.verify(token)
+assert verified_token_object.name == 'Zaphod Beeblebrox'
+```
+
+The only difference is that you do not have to pass the key for encoding or decoding the token to the constructor and
+`verify()` method, respectively (you still can do so if you do not want to use the default key defined in your
+application's configuration).
+
+Additionally, if the configuration value `EASYJWT_TOKEN_VALIDITY` is set, the token will
+be valid for the amount specified in this configuration value after it has been created with `create()`. If this
+configuration value is not set tokens will not expire. If you explicitly set the expiration date on a token object
+this value will always take precedence (if it is not `None`):
+
+```python
+import datetime
+
+from flask_easyjwt import FlaskEasyJWT
+from flask import Flask
+
+# Define the claims of your token.
+class MySuperSimpleJWT(FlaskEasyJWT):
+
+    def __init__(self, key):
+        super().__init__(key)
+
+        # Define a claim `name`.
+        self.name = None
+
+# Define the default configuration options for FlaskEasyJWT
+# in the configuration of your Flask app.
+app = Flask(__name__)
+app.config.from_mapping(
+    EASYJWT_KEY='Super secret key',
+    EASYJWT_TOKEN_VALIDITY=datetime.timedelta(minutes=7)
+)
+
+# Assuming we are within a Flask app context.
+
+token_object = MySuperSimpleJWT()
+token_object.name = 'Zaphod Beeblebrox'
+
+# This token will expire in 15 minutes, even though the default token validity is set to 7 minutes.
+token_object.expiration_date = datetime.datetime.utcnow() + datetime.timedelta(minutes=15)
+```
+
+Initializing token objects and creating and verifying tokens must be executed within a
+[Flask application context](https://flask.palletsprojects.com/en/1.1.x/appcontext/) if you want to use the configuration
+values from the application's configuration.
+
 ## Acknowledgements
 
 Flask-EasyJWT is just an easy-to-use abstraction layer around José Padilla's