Fix readme commands for entering docker environment on linux

[davidcallanan]

This resolves issue #1: no rule to make target

The commands should be the following when entering the docker build environment:

• Linux: docker run --rm -it -v "$(pwd)":/root/env myos-buildenv
• MacOS: docker run --rm -it -v "$(pwd)":/root/env myos-buildenv
• Windows (CMD): docker run --rm -it -v "%cd%":/root/env myos-buildenv
• Windows (PowerShell): docker run --rm -it -v "${pwd}:/root/env" myos-buildenv

If you are using git bash, WSL, or msys2 on windows, you'll probably need to use the linux command above.

[bonelesschickenbone]

When I put in 'make build-x86_64' it always says 'makefile:5:*** missing separator. Stop.'

[davidcallanan]

Changes merged to master.

[ghost]

It still give me the same error "make *** No rule to make target 'build x86_64' . stop."