Merge pull request #78 from CiscoDevNet/readme-preview

Add preview section

Author: charleslavoy

README.md

@@ -16,6 +16,22 @@ These Learning Labs are for building your home networking lab for network progra
 
 If you need more help, reach out to DevNet through one of our [support options](https://developer.cisco.com/site/devnet/support/).
 
+## Preview Learning Lab Markdown locally
+
+You can preview how the Markdown renders by using a pre-built Docker image. The `Makefile` in the root of the repository lets you run `make preview` to view the output HTML.
+
+1. Make sure you have Docker installed locally. If not, [install Docker](https://docs.docker.com/install/) for your operating system.
+```
+$ docker -v
+```
+1. In the root of the repository, run:
+```
+$ make preview
+```
+1. Open a browser window with the URL: `http://localhost:9000`.
+1. Click a folder to find the Markdown file you want to preview.
+1. When you are done previewing, type `Ctrl+C` to stop running the Docker container.
+
 ## Contributing
 
 These learning modules are for public consumption, so you must ensure that you have the rights to any content that you contribute.