minor #147 [UPGRADE] Describe v3 to v4 upgrade (lchrusciel)

This PR was merged into the 4.0-dev branch.

Discussion
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| Q               | A
| --------------- | ---
| Bug fix?        | no
| New feature?    | no
| BC breaks?      | no
| Related tickets | 
| License         | MIT

Commits
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39b19b9 [UPGRADE] Describe v3 to v4 upgrade

UPGRADE.md

@@ -1,5 +1,10 @@
 # ApiTestCase upgrade instructions
 
+## Upgrading from v3 to v4
+
+* Change namespace from `Lakion\ApiTestCase` to `ApiTestCase``
+* If you are using `polishsymfonycommunity/symfony-mocker-container`, you need to whitelist it as your requirement
+
 ## Upgrading from v2 to v3
 
 * JSON output is not escaped any more when comparing it to expected data. Turn on `ESCAPE_JSON` flag in your test configuration retain previous behaviour.