bendroid: Add a README.md
bendroid: Add a README.md

In particular, stress on what needs to be done to use bendroid when one
is not a Vanadium contributor (and thus may not have "jiri" or the
"android profile" installed)

Change-Id: Ib74cd536851ac802703e47d1811668b1cebb7e5b

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools