Gathering information about test filter files into a README.md file.

testing/buildbot/filters/README.md

+# Test filter files
+
+## Summary
+
+This directory contains files that list tests that are not yet ready to run in a
+particular mode. For example - the `site-per-process.browser_tests.filter` file
+lists tests that should be excluded when running `browser_tests` in
+`--site-per-process` mode.
+
+## File syntax
+
+Contents of test filter files follow the syntax below:
+
+-   Empty lines are ignored
+-   Lines starting with the '#' character are treated as comments and ignored.
+-   All other lines specify a single [test name pattern][gtest_filter].
+    -   Negative patterns: Patterns prefixed with the '-' character specify
+        tests to exclude from a test run.
+    -   Positive patterns: All other patterns specify tests to include in a test
+        run.
+
+A test will be executed if and only if it matches any of the positive patterns
+but does not match any of the negative patterns (please see documentation of
+[--gtest_filter][gtest_filter] for more details and examples).
+
+Please see [https://crbug.com/587527] for discussion how "positive" and
+"negative" test patterns should be combined in presence of both the
+`--gtest_filter` and the `--test-launcher-filter-file` command line flags.
+
+Example test filter file for excluding a set of tests:
+
+```test.filter
+# crbug.com/417518: Get tests working w/ --site-per-process
+-BrowserTest.OtherRedirectsDontForkProcess
+-ChromeRenderProcessHostTest.*
+-ReferrerPolicyTest.HttpsRedirect
+
+# crbug.com/448592: Get extension browsertests working w/ --site-per-process
+-IsolatedAppTest.CookieIsolation
+-IsolatedAppTest.CrossProcessClientRedirect
+-IsolatedAppTest.IsolatedAppProcessModel
+-IsolatedAppTest.SubresourceCookieIsolation
+```
+
+## Usage
+
+When running tests on desktop platforms, the test filter file can be specified
+using `--test-launcher-filter-file` command line flag. Example test invocation:
+
+```bash
+$ out/dbg/content_browsertests \
+    --site-per-process \
+    --test-launcher-filter-file=testing/buildbot/filters/site-per-process.content_browsertests.filter
+```
+
+When running tests on Android, the test filter file can be specified using
+`--gtest-filter-file` command line flag. Example test invocation:
+
+```bash
+$ out/android/bin/run_content_browsertests \
+    --test-arguments=--site-per-process \
+    --gtest-filter-file=testing/buildbot/filters/site-per-process.content_browsertests.filter
+```
+
+## Applicability
+
+Test filter files described here are currently only supported for gtest-based
+tests.
+
+For excluding layout tests when running with a particular command line flag, see
+`third_party/WebKit/LayoutTests/FlagExpectations/README.txt`.
+
+## Adding new test filter files
+
+Please use the following conventions when naming the new file:
+
+-   Please include the name of the test executable (e.g.
+    `content_browsertests`).
+-   Please use `.filter` suffix.
+-   Feel free to add other relevant things into the file name (e.g. the mode the
+    file applies to - for example `site-per-process`).
+
+When adding a new file, please update `//testing/buildbot/filters/BUILD.gn`.
+
+[gtest_filter]: https://github.com/google/googletest/blob/master/googletest/docs/AdvancedGuide.md#running-a-subset-of-the-tests