Create a README.md for the history component.

And adds basic explanation on the differences between the different
history services it houses.

Change-Id: Ic93603c9b2d9312315886a0fe10a4940c8b44829
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495907
Auto-Submit: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821891}

components/history/README.md

+# History Component
+
+Component that manages visited URLs history.
+
+## History services
+
+There are 3 different history services that fulfill different requirements:
+
+* [HistoryService](/components/history/core/browser/history_service.h): manages
+and allows access to local-only history.
+* [WebHistoryService](/components/history/core/browser/web_history_service.h):
+handles queries to history servers providing overall history access (local and
+remote), potentially including from other devices synced to the same account.
+* [BrowsingHistoryService](/components/history/core/browser/browsing_history_service.h):
+is a layer on top of the two services above and
+[SyncService](/components/sync/driver/sync_service.h) that works transparently
+for both sync'ing and non-sync'ing users.