Issue 003: Add GPL v3 licence reference topic

Source: Plain text version of GPL v3 and images from
http://www.gnu.org/licenses/gpl.html

Notes:
- Only available as en-us language version
- Replicated as closely as possible to the HTML version
- Used section ids from DocBook version
- For US english, license is spelt the same for both the noun and verb,
in UK english the noun is spelt as licence and the verb as license