Add UPGRADE.md (#82)

Co-authored-by: Alexander Makarov <[email protected]>

UPGRADE.md

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+# Upgrading Instructions for Yii Runner HTTP
+
+This file contains the upgrade notes. These notes highlight changes that could break your
+application when you upgrade the package from one version to another.
+
+> **Important!** The following upgrading instructions are cumulative. That is, if you want
+> to upgrade from version A to version C and there is version B between A and C, you need
+> to following the instructions for both A and B.
+
+## Upgrade from 2.x
+
+- Pass PSR-3 compatible logger to `HttpApplicationRunner` to log errors while container is building.
+- Remove `ServerRequestFactory` and `SapiEmitter` usage from your code.