documentation improvements

Motivated by a couple spelling errors and having remembered that it was really hard to understand the install/configuration procedures for Multilingual Support, I set out to contribute some improvements to the README.txt file for upcoming d6 release. I hope you find my changes useful.

Changes include:

• Improved consistency in terminology:
  • "Simplenews" throughout, instead of "simplenews and Simplenews" except where necessary (eg, to denote lowercase folder name).
  • Standardized terminology between vocabulary and node representations of newsletters. Used "Newsletter issue" to represent a newsletter node, and "Newsletter" or "Newsletter series" (for added clarity where necessary).
• Improved explanation, sentence structure, and duplicate info in the DESCRIPTION section.
• Improved explanations in the MULTILINGUAL SUPPORT section.
• Simplified the SENDING NEWSLETTERS WITH CRON section.
• Improved explanations of how Simplenews uses the Taxonomy module.
• Stripped trailing whitespace and converted tabs to spaces.
• Enforced word wrap at 80 columns.