Add screen orientation spec to API list

Fixes https://crosswalk-project.org/jira/browse/XWALK-1296
Add link to download page to explanation of release numbers

Fixes https://crosswalk-project.org/jira/browse/XWALK-1277
Merge pull request #92 from townxelliot/minor-edits

Minor edits
Add note to FAQ about support for desktop platforms
Update build instructions

This addresses https://crosswalk-project.org/jira/browse/XWALK-1307
and https://crosswalk-project.org/jira/browse/XWALK-1059.