minor #608 Improve documentation (lchrusciel)

This PR was merged into the 1.0-dev branch.

Discussion
----------

#SymfonyHackday

Commits
-------

03acf8a Improve documentation

Author: lchrusciel

CHANGELOG.md

@@ -10,6 +10,7 @@
 
 ### Added
 - [#593](https://github.com/Sylius/SyliusShopApiPlugin/issues/593) [Checkout] Send confirmation email after checkout is completed ([@lchrusciel](https://github.com/lchrusciel))
+- [#599](https://github.com/Sylius/SyliusShopApiPlugin/issues/599) Validate complete order request ([@kayue](https://github.com/kayue), [@mamazu](https://github.com/mamazu))
 
 ### Changed
 - [#590](https://github.com/Sylius/SyliusShopApiPlugin/issues/590) [Maintenance] Register handler only in proper bus ([@lchrusciel](https://github.com/lchrusciel))

UPGRADE.md

@@ -1,3 +1,15 @@
+# UPGRADE FROM 1.0.0-rc.2 or 1.0.0-rc.3 to 1.0.0 
+
+* Add proper constraint to composer.json
+```json
+{
+    "require": {
+        "sylius/shop-api-plugin": "^1.0"
+    }
+}
+```
+* Run `composer update`
+ 
 # UPGRADE FROM 1.0.0-rc.1 to 1.0.0-rc.2
 
 * The following files changed their namespaces and folders: