create update file

UPDATE.md

@@ -0,0 +1,8 @@
+#How to update your Thelia
+
+- copy all files from the thelia new version (local/modules/* files too)
+- clear all caches running ```php Thelia cache:clear```
+- run ```php Thelia thelia:update```
+- again clear all caches in all environment :
+    - ```php Thelia cache:clear```
+    - ```php Thelia cache:clear --env=prod```