Documentation and messages suggestions

Hi Matteo,

I've just started to use slackpkg+ and it is awesome!

To say 'thank you' I have tried to review all the documentation and messages (e.g. echo commands) for typos, spelling etc.  Then I did a big reformat on src/README, I hope you like it, but it's not really different to your version - you already wrote everything important, and in the right order too!

If you don't use any of these suggestions that is cool!  I haven't suggested any changes to the code, because it's perfect! :)

Thanks for a great addition to Slackware's tools - I think slackpkgplus will improve the whole ecosystem, because it makes it practical for people to use repositories in the same way as Ubuntu PPA's etc.  (Sorry to mention Ubuntu ;-)

Best wishes
-Dave Spencer