Update Devtools README.
Update Devtools README.

This drops the section about devtoolslib, which became an implementation
detail. mojo_* top-level tools are now *the* supported way of consuming
devtools.

R=qsr@chromium.org

Review URL: https://codereview.chromium.org/1265243002 .

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