Add more detail to web_platform_tests.md about manual tests

Change-Id: I025016b77d66ef7f34ff24a6bc3b0d2df046ce05
Reviewed-on: https://chromium-review.googlesource.com/560089
Reviewed-by: Philip Jägenstedt <[email protected]>
Commit-Queue: Quinten Yearsley <[email protected]>
Cr-Commit-Position: refs/heads/master@{#484404}
1 file changed