Added .arcignore

Author: alexey-milovidov

.arcignore

@@ -0,0 +1,12 @@
+# .arcignore is the same as .gitignore but for Arc VCS.
+# Arc VCS is a proprietary VCS in Yandex that is very similar to Git
+# from the user perspective but with the following differences:
+# 1. Data is stored in distributed object storage.
+# 2. Local copy works via FUSE without downloading all the objects.
+# For this reason, it is better suited for huge monorepositories that can be found in large companies (e.g. Yandex, Google).
+# As ClickHouse developers, we don't use Arc as a VCS (we use Git).
+# But the ClickHouse source code is also mirrored into internal monorepository and our collegues are using Arc.
+# You can read more about Arc here: https://habr.com/en/company/yandex/blog/482926/
+
+# Repository is synchronized without 3rd-party submodules.
+contrib