Added migraiton notes

MIGRATION.md

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+# Migraiton
+
+## 3.x
+
+### PHP-DI
+
+The biggist change is the support of php-di 6 and the dropped support of php-di 5. The migration is somwhat huge
+depending on the number of defintions. See http://php-di.org/doc/migration/6.0.html for details
+
+### API
+
+- All occurances of `Interop\Container\ContainerInterface` have been replaced with `\Psr\Container\ContainerInterface` so updated your usages accordingly
+- The types of `\de\codenamephp\platform\di\Container::__construct` have changed so if you instantiate the container yourself (you shouldn't, that's what the 
+  builder is for) you have to update the type of the first and 3rd parameter
+- All parameters of `\de\codenamephp\platform\di\Container::__construct` are now optoinal so if you just passed them because you had to, remove them
+
+### PHP
+
+PHP7 is now required for php-di 6 and type hints