update documentation to mention that slurp-config uses classpath and not a relative path

[gdeer81]

lets say you have a top level config folder with props.config in it
if you use plain old slurp like this (slurp "config/props.config") you'll get the contents of the file
but if you use (slurp-config "config/props.config") you'll get nil
Then you'll discover that slurp-config uses the classpath to find config files so you'll run'lein classpath' and see that your config directory is not in your classpath.
so you'll add :resource-paths ["config"] to your project.clj
you verify the config folder is now in your classpath, but slurp-config still returns nil
so you drop the folder name and just use (slurp-config "props.config") and it

If only the documentation had mentioned this

[gdeer81]

#2 fixes this issue