Add more text to components/services/README about some more reasons why a service is there instead of /services.

Change-Id: I93b26022d0391ef3daa18d9b21ee55777eebfe34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610537
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Auto-Submit: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659469}