add example of dependency wrapping

[briceburg]

dex can run more than applications -- it can run arbitrary commands within a container containing dependeices; e.g.

$ mkdir -p test/{a..c}
$ ls test/
a  b  c

# run ls within a container
$depswrap ls test/
a  b  c

node example

$ node -v
bash: node: command not found

$ depswrap node -v
v6.7.0

$ depswrap npm install 
test@ /dex/workspace/home/nesta/tmp/test
`-- json-lint@0.1.0

# local machine gets node_modules
$ ls -l node_modules
total 4
drwxr-xr-x 5 nesta nesta 4096 Nov 16 11:06 json-lint