Update README.txt: version 3.0 has new dependancies and different modules than 2.0

I've made it more clearer in the release notes as well as changed the Readme.txt file.

The README.txt file needs an overhaul for 3.0

1) Requirements section might want to mention libraries module, and the jQuery cycle plugin, as well as views 3.

2) Top of the readme file is now out of date, still refers to single frame and thumbnail hover modules, which are gone.

3) There is no mention anywhere of what to do if you updated your module from 2.x:

All the views on my site that were using the slideshow style were broken after updating from the 2.x branch to 3.x. The module I was previously using (single frame) was suddenly missing. After realizing that I needed the libraries API module for the 3.x version of vies slideshow, I was unable to turn it on since views slideshow was still enabled (see #1366848: Add an update hook for upgrading from 2.x to 3.x). After enabling the libraries API module, everything remained broken because the new module "Views Slideshow: Cycle" also needed to be enabled. Even after enabling the new module, I wasn't able to get a slideshow. I was able to *guess* that since this new module has the word "Cycle" in the title, and because I needed libraries API to use it, that I might also need to go and download the jQuery cycle plugin and add it to my "libraries" directory, but there's nothing anywhere (on my site, or in the documentation) that tells me to do that (see #1366850: Notify users of missing jQuery cycle plugin via hook_requirements)

The views slideshow project page tells people to read the documentation for installation instructions, but the documentation page is almost empty.

I updated the documentation for installation and upgrading.

And as promised the readme files are now updated. Thank you very much for doing the docs on this!

Thanks :)