Add additional info to help new users

DOCKER_README.md

@@ -5,10 +5,28 @@ Minimal docker environment for building arm-embedded projects
 Run interactively with ```docker run -it --rm -v `pwd`:/root ryankurte/docker-arm-embedded /bin/bash```.  
 This will create a temporal instance (changes will be dropped on exit) with a binding from the current directory to the root user home directory.  
 
+If you are using selinux you may need to add a ```:z``` to the end of the -v parameter to indicate the mount is shared.
+
+```docker run -it --rm -v `pwd`:/root:z ryankurte/docker-arm-embedded /bin/bash```
+
+https://docs.docker.com/storage/bind-mounts/
+
+### Building grbl-LPC
+Update the grbl/config.h and uncomment the CPU_MAP define statement for the board you are compiling for
+
+```
+git submodule init
+git submodule update
+make
+```
+
+The resulting firmware file is build/firmware.bin
+
 ## Includes:
  - build-essential (native)
  - make, cmake
  - gawk, genromfs, ccache
  - arm-none-eabi from [launchpad.net](https://launchpad.net/~terry.guo/+archive/ubuntu/gcc-arm-embedded)
  - [Yotta](http://yotta.mbed.com/)
- 
+
+