[Downloads] Add Android/stable/ARM link
Merge pull request #90 from townxelliot/android-stable-version-1308

Add Android/stable/ARM link to Downloads table
Add link to SIMD API spec
Merge pull request #91 from townxelliot/simd-api-docs-1237

Add link to SIMD API spec
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.