add a README for new test directory

src/ruby/end2end/README

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+This directory contains some grpc-ruby end to end tests.
+
+Each test here involves two files: a "driver" and a "client". For example,
+the "channel_closing" test involves channel_closing_driver.rb
+and channel_closing_client.rb.
+
+Typically, the "driver will start up a simple "echo" server, and then
+spawn a client. It gives the client the address of the "echo" server as
+well as an address to listen on for control rpcs. Depending on the test, the
+client usually starts up a "ClientControl" grpc server for the driver to
+interact with (the driver can tell the client process to do strange things at
+different times, depending on the test).
+
+So far these tests are mostly useful for testing process-shutdown related
+situations, since the client's run in separate processes.
+
+These tests are invoked through the "tools/run_tests/run_tests.py" script (the
+Rakefile doesn't start these).