Remove '.sh' from scalelite_prune_recordings such that it actually gets executed by cron

[Ithanil]

Having a dot in a filename prevents execution by cron when put under /etc/cron directories. Therefore, this PR removes the .shextension from the filename. Also, a short README section is added.

See https://github.com/blindsidenetworks/scalelite/issues/1044 for further details regarding the cron behavior.

Fixed issues

https://github.com/blindsidenetworks/scalelite/issues/1044

Note

This will break any automated build scripts relying on the old filename to download the script from the repo. However, I assume in many cases they didn't rename the file and thus the script didn't do anything anyway.

[ffdixon]

Thanks @Ithanil - much appreciated.