Update README.md.

The `ResourceTest` mentioned in the doc has been moved into
recovery_component_test in commit
6b28f05c.

Since Q, `/sbin/adbd` has been relocated to `/system/bin/adbd`. And `adb
shell` no longer requires mounting /system first, because
`/system/bin/sh` has been included in recovery image.

Test: N/A
Change-Id: I72029a6c8bfaac7a17c0a526a3c511de19356baa