Chapter 1 fixes

- general spelling/grammar
- add Mac OS X info
Merge pull request #23 from rthorp/FIX-Chapter1

Chapter 1 fixes
Update readme
Make warning clearer
Merge branch 'master' of https://github.com/gitextensions/GitExtensionsDoc
Readme fix
Typo
Merge branch 'master' into latest
Changed URL location to officiall site.
Merge branch 'master' into latest