Add initial README content (#15)

README.md

@@ -11,3 +11,42 @@ Install through Composer:
 ```bash
 composer require --dev lendable/phpunit-extensions
 ```
+
+## Features
+
+### Strict mocking
+
+By default, when creating a mock all method return values are stubbed based on typing information. E.g.
+
+```php
+public function foo(): int 
+{
+    return $this->foo;
+}
+```
+
+Will be stubbed to return `0`. This library ships two mechanisms to simplify disabling this functionality to force all methods called on a mock to be configured explicitly.
+
+* If you extend from `Lendable\PHPUnitExtensions\TestCase`, this will be enabled. If you can just extend from this class it is the simplest way to opt-in to all functionality.
+* The trait `Lendable\PHPUnitExtensions\StrictMocking` is provided to enable this alone. If you are forced into using another abstract `*TestCase` (e.g. from a vendor) this can be added into the class hierarchy.
+
+## PHPStan
+
+A PHPStan extension is provided to enforce usage of features of this library.
+
+Add the rules into your PHPStan configuration:
+
+```neon
+rules:
+    # ...
+    - vendor/lendable/phpunit-extensions/phpstan/rules.neon
+```
+
+Configure any exclusions you may have:
+
+```neon
+lendable_phpunit:
+    enforceStrictMocking:
+        pardoned:
+            - Foo\Bar\MyTest
+```