Updated installation instructions for OSX
typos
Initial contributing documentation
Merge pull request #42 from miohtama/osx-installation-instructions

Updated installation instructions for OSX
Added gitbook screenshot
Finalizing words of Contributing chapter
Updated README and SUMMARY to link to contributing
Some typo fixes, link updates.
Merge pull request #44 from kampfschlaefer/typos

Thanks!
wording, sentence structure
Merge pull request #45 from miohtama/contributing

Thanks. Content looks OK, but it would be nice to have it proof read by some English experts.
Merge branch 'master' of github.com:DjangoGirls/tutorial
Folder structure rejig
missed 2 venv tags
Merge pull request #49 from BluesFiend/master

Folder structure rearrange
wording, grammar, clarified explanations
Merge branch 'master' of github.com:DjangoGirls/tutorial
Merge pull request #48 from kerstin/master

Thanks
Changing default db to sqlite. Moving postgresql tutorial to optional section.
updating summary page