Add README with instructions for running Nav2 and SLAM (#126).

navigation2/README.md

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+# Navigation2 Docker Image
+
+The Navigation2 Docker image uses the Space ROS docker image (*osrf/spaceros:latest*) as its base image.
+The Navigation2 Dockerfile installs all of the prerequisite system dependencies to build Navigation2
+## Building the Navigation2 Image
+
+To build the docker image, run:
+
+```
+./build.sh
+```
+
+The build process will take about 30 minutes, depending on the host computer.
+
+## Running the Navigation2 Docker Image in a Container
+
+After building the image, you can see the newly-built image by running:
+
+```
+docker image list
+```
+
+The output will look something like this:
+
+```
+REPOSITORY              TAG                        IMAGE ID       CREATED        SIZE
+osrf/space_nav2         latest                     6edb2edc9643   10 hours ago   15.5GB
+openrobotics/spaceros   latest                     629b13cf7b74   12 hours ago   7.8GB
+nvidia/cudagl           11.4.1-devel-ubuntu20.04   336416dfcbba   1 week ago     5.35GB
+```
+
+The new image is named **osrf/space_nav2:latest**.
+
+There is a run.sh script provided for convenience that will run the spaceros image in a container.
+
+```
+./run.sh
+```
+
+Upon startup, the container automatically runs the entrypoint.sh script, which sources the MoveIt2 and Space ROS environment files.
+You'll now be running inside the container and should see a prompt similar to this:
+
+```
+root@ip-your-ip-address:/home/spaceros-user/nav2_ws#
+```
+
+## Running Navigation2 Demo
+
+To run the latest demo, see the README in the nav2_demos folder