Current file generation approach is broken with recent leiningen

[Julien]

Starting 2.4.2 leiningen calls the clean task right before the deploy task. This implies for most cljx configuration jars uploaded won't contain any generated file. Starting 2.5.0 the same happens before install so even local jars won't be properly generated.
:auto-clean false can be used for this but it looks like the preferred solution is to rely on :prep-tasks "cljx once".

Documentation should probably be updated as this behavior is pretty unexpected.

See leiningen#1642 and leiningen#1586 for more details.

[Julien]

This is related to #49.

[Chas Emerick]

A doc PR to change the recommended setup is welcome. My time for OSS work is severely limited these days.

[Julien]

Sure! I didn't do it before as I wasn't sure if cljx should recommend a specific leiningen version. I figured it is simpler to just add a documentation for newer leiningen.

#56 should address that.