[sdw] [ssn] editing ssn deliverable on github

Following decisions made at the ssn meeting today, I have updated the main page on the wiki https://www.w3.org/2015/spatial/wiki/Main_Page
with advice to ssn editors and all other participants in sdw to make changes to the SSN Editor's Draft (following the approach used for the BP deliverable).

SSN Editor's Draft on github: http://w3c.github.io/sdw/ssn/

How to edit here: https://www.w3.org/2015/spatial/wiki/How_to_work_with_GitHub_for_ssn

--Kerry