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Add a README.md for mojo core.

This is the document I wish I could have read rather than all the
implementation. Maybe this will save others time in the future.

Change-Id: I1858808ba4cce584625325fea383a5b2332da395
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1635414Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666338}