Add description related to using docker

[Taeyoung96]

@gaoxiang12
I add simple comments related to using Docker.
When I follow the readme and build the environment with Docker, I faced the following error.

fasterlio-ubuntu20_1  | [100%] Built target run_mapping_offline
fasterlio-ubuntu20_1  | ================Docker Env Ready================
fasterlio-ubuntu20_1  | No protocol specified
fasterlio-ubuntu20_1  | Unable to init server: Could not connect: Connection refused
fasterlio-ubuntu20_1  | 
fasterlio-ubuntu20_1  | (lxterminal:1): Gtk-WARNING **: 11:18:47.105: cannot open display: :0

The solution is quite simple, just enter xhost +local:docker in the local terminal, before entering docker-compose up.
I thought writing the solution on the readme would be one way for beginners to avoid confusion.

Thanks,

[gaoxiang12]

Thanks for the PR! I'm not so familiar with docker.