Add rebar3 tool files and directories (#3178)

* Add rebar3 tool files and directories

* Add headers to specify tools artifacts

Erlang.gitignore

@@ -1,10 +1,17 @@
 .eunit
-deps
 *.o
 *.beam
 *.plt
 erl_crash.dump
-ebin/*.beam
-rel/example_project
 .concrete/DEV_MODE
+
+# rebar 2.x
 .rebar
+rel/example_project
+ebin/*.beam
+deps
+
+# rebar 3
+.rebar3
+_build/
+_checkouts/