Document Appsody "airgap" (appsody-nginx)

[chilanti]

Is your feature request related to a problem? Please describe.
When access to the internet (GitHub) is not guaranteed, we have a stop gap solution that allows a developer to work in isolation (at least for appsody list and appsody init) - but that solution needs to be documented.

Describe the solution you'd like
Documentation on downloading the local nginx image and using it.
The steps are as follows:

1. Make sure your 8008 port is available locally:
   netstat -na |grep 8008
   (or pick another port)

2. Download the nginx image:
   docker pull appsody/appsody-index

3. Run it:
   docker run -p 8008:8080 -e EXTERNAL_URL=http://localhost:8008/ appsody/appsody-index

4. Check that someone is listening on 8008:
   sudo lsof -PiTCP -sTCP:LISTEN

5. Add the new repo and make it the default:
   appsody repo add nginx-incubator http://localhost:8008/incubator-index.yaml
appsody repo set-default nginx-incubator

6. Remove repos that aren't accessible:
   appsody repo remove <repo name>
   for every repo.

7. Now run appsody list.

[skoh7645]

Users won't need to do the "remove repos that aren't accessible" step, once appsody/appsody#308 goes in