README: revamp the "Run Mojo Shell" section.
README: revamp the "Run Mojo Shell" section.

This patch rewrites instructions for running Mojo shell basing the
examples on the `mojo_shell.py` + `debugger` workflow.

R=tonyg@chromium.org

Review URL: https://codereview.chromium.org/1170283002.

jianglu/mojo,chinmaygarde/mojo,jianglu/mojo,chinmaygarde/mojo,afandria/mojo,jianglu/mojo,chinmaygarde/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,afandria/mojo,jianglu/mojo,afandria/mojo,afandria/mojo,jianglu/mojo,chinmaygarde/mojo,afandria/mojo,chinmaygarde/mojo,afandria/mojo,jianglu/mojo,afandria/mojo,chinmaygarde/mojo,chinmaygarde/mojo,chinmaygarde/mojo,chinmaygarde/mojo