[test] Add README to test/mkgrokdump, containing rebaseline instruction.

TBR=yangguo@chromium.org

Bug: 
Change-Id: I7407b394a0e1f6a3ffdfe5a9a5a123c30ad3e5c6
Reviewed-on: https://chromium-review.googlesource.com/506196
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45347}

test/mkgrokdump/README

+If you change the heap layout, expect this test to fail (in CQ). You will then
+need to build x64 release and run `<outdir>/mkgrokdump > tools/v8heapconst.py`
+to rebaseline.