Update docs for setting env variables

The documentation contained no information about how to escape strings
when setting an environment variable on an application to update the
buildpack configuration. This change adds an example.

Author: Christopher Frost

README.md

@@ -31,6 +31,10 @@ Buildpack configuration can be overridden with an environment variable matching
 
 ```cf set-env my-application JBP_CONFIG_OPEN_JDK_JRE '[version: 1.7.0_+, memory_heuristics: {heap: 85, stack: 10}]'```
 
+If the key or value contains a special character such as `:` it should be escaped with double quotes. For example, to change the default repository path for the buildpack.
+
+```cf set-env my-application JBP_CONFIG_REPOSITORY '[ default_repository_root: "http://repo.example.io" ]'```
+
 Environment variable can also be specified in the applications `manifest` file. See the [Environment Variables][] documentation for more information.
 
 To learn how to configure various properties of the buildpack, follow the "Configuration" links below. More information on extending the buildpack is available [here](docs/extending.md).