add instruction about docker image into README

Created by: garasubo

Problem: [describe the problem you try to solve with this PR.] In some Linux distribution, it is hard to install redox's toolchain.

Solution: [describe carefully what you change by this PR.] I think it is kind if README tell us the way to setup with docker.

Fixes: [what issues this fixes.] #970 (closed)

State: [the state of this PR, e.g. WIP, ready, etc.] ready