CONTRIBUTING.md: Complete rewrite of CONTRIBUTING

- Add some documentation from the archived OpenFlow website, found
  here:

      http://archive.openflow.org/wk/index.php/OFTestTutorial

- Add some documentation from the OpenFlow Hub website, found here:

      http://www.openflowhub.org/display/OFTest

- Reformat as Markdown
- Remove old or irrelevant sections
- Rename 'doc' to 'docs'
- Merge 'STYLE' into 'CONTRIBUTING.md', as this is expected by GitHub:

      https://github.com/blog/1184-contributing-guidelines

- Add some additional information on contributing to OFTest.

This still includes TODOs from the original sources. These will need to
be resolved in a future commit.

Signed-off-by: Stephen Finucane <stephenfinucane@hotmail.com>