Clarify mojo/devtools/README.md.
Clarify mojo/devtools/README.md.

State it upfront that the actual devtools package sits in ./common.

R=qsr@chromium.org

Review URL: https://codereview.chromium.org/1248373002 .

afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo