Updated contribution documentation

Added a section on how issues are resolved.
Cleaned up some of the wording
small change to indent style guide bullets for comments
small edit to section header
Added suggested changes from @dglazer from issue #36

- s/they/the/
- clearer language on contribution methods
- clearer language on vote scheme
s/GA4GH ReadTaskTeam Project/GA4GH Schemas/
Merge pull request #36 from delagoya/contribution_documentation

Contribution documentation
Merge remote-tracking branch 'ga/master' into alt_method_definitions