Add upgrade documentation

Author: johnkary

UPGRADE.md

@@ -0,0 +1,21 @@
+UPGRADE FROM 1.x to 2.0
+=======================
+
+### `JohnKary\PHPUnit\Listener\SpeedTrapListener` subclasses must implement scalar type hints
+
+SpeedTrapListener was upgraded to support PHP 7 scalar type hints. Any
+subclass will need to update the overridden function signature:
+
+* Declare strict types at the top of your subclass: `declare(strict_types=1);`
+* Update method signatures:
+
+| Old signature | New signature |
+| -------- | --- |
+| `protected function isSlow($time, $slowThreshold)` | `protected function isSlow(int $time, int $slowThreshold) : bool`
+| `protected function addSlowTest(TestCase $test, $time)` | `protected function addSlowTest(TestCase $test, int $time)`
+| `protected function hasSlowTests()` | `protected function hasSlowTests() : bool`
+| `protected function toMilliseconds($time)` | `protected function toMilliseconds(float $time) : int`
+| `protected function makeLabel(TestCase $test)` | `protected function makeLabel(TestCase $test) : string`
+| `protected function getReportLength()` | `protected function getReportLength() : int`
+| `protected function getHiddenCount()` | `protected function getHiddenCount() : int`
+| `protected function getSlowThreshold(TestCase $test)` | `protected function getSlowThreshold(TestCase $test) : int`