Update the readme to mention validate.py.

Change-Id: Ib844cbe34ba755b66f752ff6f4c235cf2d45481f

README.md

@@ -127,6 +127,8 @@ packaging is handled by `build/tools/package.py`.
 Testing
 -------
 
+### Single Configuration Testing
+
 Running the NDK tests requires a complete NDK package (see previous steps).
 From the NDK source directory (not the extracted package):
 
@@ -142,6 +144,28 @@ device/emulator is connected. If you do not have a device capable of running the
 tests, you can run just the `build` or `awk` test suites with the `--suite`
 flag.
 
+### Thorough Testing
+
+There is also a script that will automatically detect connected
+devices/emulators and choose a subset of them to fit our QA configuration. This
+script will run all of the tests across all available ABIs on several devices,
+and thus will take a long time (takes ~75 minutes on my machine, even with a few
+configurations unavailable). As such, this isn't suitable for active
+development, but should be run for QA and for any changes that might have a wide
+impact (compiler updates, ndk-build changes, sysroot changes, etc).
+
+To use this script, connect any devices and launch any emulators you need for
+testing (make sure ADB has authorization to control them), then run:
+
+```bash
+$ python tests/validate.py path/to/ndk
+```
+
+By default, test logs will be placed in $CWD/test-logs. This can be controlled
+with the `--log-dir` flag.
+
+### Testing libc++
+
 The libc++ tests are not currently integrated into the main NDK tests. To run
 the libc++ tests: