Include README description.

README.md

@@ -1,3 +1,44 @@
 # Overview
 
-Flexceptions (Flexible Exceptions) is an error handling library for Python3.
+Flexceptions (Flexible Exceptions) is an error handling library for Python3.
+
+### Description
+Flexceptions provides a hierarchy of exceptions with a handling mechanism. This allows defining a specific "handling" behaviour for the exception and delegating the execution of that behaviour to a later point.
+
+A handler is no more than any function conforming to the `Handler` typing.
+```python
+from flexceptions import Handler
+# Handler = Callable[["BaseFlexception"], HandlerReturn]
+```
+
+You can store a handler to be executed upon calling the public `handle` method. There are three ways to set a handler in a flexception:
+```python
+from flexceptions import BaseFlexception, Handler
+
+def my_callback(_exception: BaseFlexception) -> None:
+    ...
+
+# Class
+class MyFlexception(BaseFlexception):
+    DEFAULT_HANDLER: Handler = my_callback
+
+# Instance
+flexception: BaseFlexception = BaseFlexception(handler=my_callback)
+
+# Call
+BaseFlexception().handle(my_callback)
+```
+
+There's a priority hierarchy to know what handler to use:
+* If the handler parameter is passed when calling the `handle` method, it takes priority over the rest.
+* If the handler parameter is not passed when calling the `handle` method:
+  * If a handler was passed on exception instantiation, that's the one that will be used.
+  * Otherwise, it will default to the class' default handler.
+
+# Requirements
+
+* Python (>=3.8)
+
+# Installation
+
+`pip install flexceptions`