Add documentation about how to use Spray compression

See https://groups.google.com/forum/#!msg/spray-user/gJBwvtNRI9o/gXnBFvTquT4J

Currently the Spray docs indicate "spray supports the compression and decompression of HTTP requests and responses", but don't mention that this isn't enabled by default, but must be explicitly requested.


There isn't currently a good place explaining how to use this. See discussion at https://groups.google.com/forum/#!msg/spray-user/gJBwvtNRI9o/gXnBFvTquT4J

I haven't created one here, sorry, but I think this note is better than nothing.