[docs] Mention ffmpeg and dav1d in llvm-test-suite

Since llvm/llvm-test-suite#182 and
llvm/llvm-test-suite#188, these projects
can now be added as external projects within llvm-test-suite.

Author: mstorsjo

llvm/docs/TestSuiteGuide.md

@@ -367,6 +367,18 @@ For the SPEC benchmarks you can switch between the `test`, `train` and
 `ref` input datasets via the `TEST_SUITE_RUN_TYPE` configuration option.
 The `train` dataset is used by default.
 
+In addition to SPEC, the multimedia frameworks ffmpeg and dav1d can also
+be hooked up as external projects in the same way. Adding them doesn't
+add the respective projects' full testsuites though. But by including
+them in llvm-test-suite, a lot more of potentially vectorizable code
+gets compiled - which also can catch compiler bugs merely by triggering
+code generation asserts. There is a small code correctness test that compares
+the output of the compiler generated functions against handwritten assembly
+versions. (On x86, this requires the nasm tool.) The projects also contain
+microbenchmarks for measuring the performance of some functions. See
+the `README.md` files in the respective `ffmpeg` and `dav1d` directories
+under `llvm-test-suite/External` for further details.
+
 
 Custom Suites
 -------------