Add small doc about docker files and info about all images

docker/README.md

+## ClickHouse Dockerfiles
+
+This directory contain Dockerfiles for `clickhouse-client` and `clickhouse-server`. They updated each release.
+
+Also there is bunch of images for testing and CI. They listed in `images.json` file and updated on each commit to master. If you need to add another image, place information about it into `images.json`.

docker/images.json

+{
+    "docker/packager/deb": "yandex/clickhouse-deb-builder",
+    "docker/packager/binary": "yandex/clickhouse-binary-builder",
+    "docker/test/compatibility/centos": "yandex/clickhouse-test-old-centos",
+    "docker/test/compatibility/ubuntu": "yandex/clickhouse-test-old-ubuntu",
+    "docker/test/integration": "yandex/clickhouse-integration-test",
+    "docker/test/performance": "yandex/clickhouse-performance-test",
+    "docker/test/pvs": "yandex/clickhouse-pvs-test",
+    "docker/test/stateful": "yandex/clickhouse-stateful-test",
+    "docker/test/stateless": "yandex/clickhouse-stateless-test",
+    "docker/test/unit": "yandex/clickhouse-unit-test",
+    "dbms/tests/integration/image": "yandex/clickhouse-integration-tests-runner"
+}