README: spell LibreOffice correctly and add some instructions.
Presentation: update based on feedback Sven Guckes.

Sven was so kind to send me an email with a lot of feedback. I tried to
get everything in:
- (don't panic) Move :e! to own line.
- (vim modes) Fix alignment.
- Replace <command> with COMMAND everywhere.
- Replace Dutch words with English ones.
- (exercise) Remove dashes and replace "an actual dot" with "a dot".
- (motions) Replace description of "I" with "1st non whitespace
  character".
- (more motions) Remove ctrl+pgup/down.
- (more motions) Add info about paragraphs.
- (more motions) Replace "center on cursor" with "center view on current
  {object}". More on this is added to README.md.
- Insert extra slide about BEW commands (up-to-date is strange with
  bew).
- (Vim in other software) Add Ksh and Zsh.
- Replace irc info with link.