Troubleshooting Ports are not available error

[DecimalTurn]

Added the following to the README :

Troubleshooting:

If you run into an error saying "Ports are not available", it might be because port 80 is already in use on your computer. In that case, you can change the first port number in "80:80" to an available port, eg.: "81:80". Then you'll need to access the container via http://localhost:81/.

[jai-dewani]

This is something I faced as well while setting up the docker/getting-started container. Adding this sort of information would be useful, since this is an introduction documentation for docker.

CC: @mikesir87