[chromedriver] Add README

chrome/test/chromedriver/README.txt

+This file contains high-level info about how ChromeDriver works and how to
+contribute.
+
+=====Architecture=====
+ChromeDriver is an implementation of the WebDriver standard,
+which allows users to automate testing of their website across browsers.
+
+ChromeDriver is shipped separately from Chrome. It controls Chrome out of
+process through DevTools (WebKit Inspector). ChromeDriver is a shared library
+which exports a few functions for executing WebDriver-standard commands, which
+are packaged in JSON. The WebDriver open source project maintains clients in
+various languages which can load this shared library.
+
+For internal use, a custom python client for ChromeDriver is available in
+chromedriver.py, which works for desktop (win/mac/linux).
+
+=====Testing=====
+There are 4 test suites for verifying ChromeDriver's correctness:
+
+1) chromedriver2_unittests (chrome/chrome_tests.gypi)
+This is the unittest target, which runs on the main waterfall on win/mac/linux
+and can close the tree. It is also run on the commit queue and try bots by
+default. Tests should take a few milliseconds and be very stable.
+
+2) chromedriver2_tests (chrome/chrome_tests.gypi)
+This is a collection of C++ medium sized tests which can be run optionally
+on the trybots.
+
+3) python tests
+These are integration tests which can be found in run_py_tests.py. They are
+run on the chromium QA bots:
+    http://build.chromium.org/p/chromium.pyauto/waterfall
+
+4) WebDriver Java acceptance tests
+These are integration tests from the WebDriver open source project which can
+be run via run_java_tests.py. They are also run on the chromium QA bots.
+See http://src.chromium.org/viewvc/chrome/trunk/deps/third_party/webdriver
+
+=====Issues=====
+Issues are tracked in chromium's issue tracker with Feature=WebDriver:
+    https://code.google.com/p/chromium/issues/list?q=feature%3Dwebdriver