My vision of structure

Previously pull request #3.

As before, treat that rather as suggestions, than ready changes.

- Reorganized README
- Removed struture of subsequent pages, identical set of links in each header/footer
- Some paragraphs reworded
- Editing workflow extended a bit, and more exposed, before version control courses
- More links between pages

General idea is to let reader choose what to read more freely, instead of forcing "one proper order". I didn't like linear order of pages, and wanted links to be better described than just 'prev' and 'next'. Now the structure is more like a graph, with 'README' as a hub and entrance point and 'Editing workflow' as a central node.

Now the order of reading should be something like:
```
    |
    V
 README
    |
    V
Intro (goals and rules)
    |
    V
Editing workflow
  /   \
  |   (optional) VCS basics
  V   V
  Setup
  /   \
  |   (optional) Git crash course
  V   V
Collaboration
```

Note that it is more top-down: description "what we need version control for" is before "what version control is".