Added documentation about testsuite

Fix #2415

scripts/update.sh

@@ -6,6 +6,6 @@ TAG=$(git describe --tags $(git rev-list --tags --max-count=1))
 rm -rf var/cache/*
 git fetch origin
 git fetch --tags
-git checkout $TAG
+git checkout $TAG --force
 SYMFONY_ENV=$ENV composer install --no-dev -o --prefer-dist
 php bin/console cache:clear --env=$ENV