Add "Fuzz Testing" entry in src/README.md

Making it easier to find instructions to add fuzz tests.

Change-Id: I3c9a285300022a0ddf11ff9003f82ee2b2377d12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1850593Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704404}

docs/README.md

@@ -173,6 +173,7 @@ used when committed.
     Capture stack traces on layout test crashes without an attached debugger
 *   [Test Descriptions](testing/test_descriptions.md) - Unit test targets that can be
     built, with associated desciptions.
+*   [Fuzz Testing](../testing/libfuzzer/README.md) - Fuzz testing in Chromium.
 *   [IPC Fuzzer](testing/ipc_fuzzer.md) - Fuzz testing of Chromium IPC interfaces.
 *   [Running Chrome tests with AddressSanitizer (asan) and LeakSanitizer (lsan)](testing/linux_running_asan_tests.md) -
     Run Chrome tests with ASAN and LSAN builds to detect addressability issues and memory leaks.