feature #890 add upgrade instructions for 2.8 (Tobion)

This PR was merged into the 2.8 branch.

Discussion
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add upgrade instructions for 2.8

Fixes https://github.com/symfony/symfony-standard/issues/882, symfony/symfony#16738, symfony/symfony#16713

Commits
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5d05253 add upgrade instructions for 2.8

Author: fabpot

UPGRADE-2.8.md

@@ -0,0 +1,29 @@
+UPGRADE FROM 2.7 to 2.8
+=======================
+
+When upgrading Symfony from 2.7 to 2.8, beware of the following changes in the
+Standard Edition:
+
+ * Assetic is not included by default anymore.
+ * It comes with a new major version of `sensio/distribution-bundle`. If you are
+   updating the bundle in your project as well, the following changes are required:
+   - The web configurator got removed. So you need to remove the `_configurator`
+     routing entry from `app/config/routing_dev.yml`.
+   - The generated `app/bootstrap.php.cache` does not include autoloading anymore.
+     So you need to add the autoloading code in your front controllers `web/app.php`,
+     `web/app_dev.php`, `app/console` and `app/phpunit.xml.dist` (bootstrap config).
+   - If have been using the Symfony 3 directory structure already, you need to
+     overwrite the cache and log directory in your `AppKernel` as it is also done
+     in Symfony 3 now (see
+     [`app/AppKernel.php`](https://github.com/symfony/symfony-standard/blob/master/app/AppKernel.php#L31-L44)).
+ * The `app/AppKernel.php` is now autoloaded via composer.
+
+You can have a look at the
+[diff](https://github.com/symfony/symfony-standard/compare/2.7...2.8)
+between the 2.7 version of the Standard Edition and the 2.8 version that
+should help you to apply the changes in your project.
+
+Additionally, we recommend to
+[add phpunit-bridge to handle deprecations](https://github.com/symfony/symfony-standard/pull/884)
+in your test suite and to ensure tests are
+[run with full error reporting](https://github.com/symfony/symfony-standard/pull/875).