Update README.txt and friends

After a while in the issue queue, I see a lot of confusion around the .txt files in Category package. The rule that "no-one reads README's" quite don't apply here, because people really need help with this large module, sometimes. Unfortunately, the texts are not really up-to-date - I believe that some of them didn't get update since 4.7.x even! According to CVS, they are 3 years old now...

So this patch is both trivial (doesn't touch single line of code), and daring of me (as English is not my native language).

I updated all the obsolete paths/locations of various administrative pages (as pointed out by several people in the queue), reworded and/or removed some obsolete mentions (like the .install files system being "new"), added a few words about Views integration (more or less to emphasize the need of taxonomy wrapper), removed the scary notice about latest Drupal HEAD being required... Additionally, I incorporated some of my experiences with upgrade, and with common misunderstandings seen in the issue queue - especially the fact, that container node mostly needs to be examined for misconfigurations, rather than administrative pages.

While it's not perfect (it's nothing like a complete rewrite), the files should be more or less acceptable for a 6.x release of a Drupal module now. Feel free to improve further :-)