Add documentation for WoT Thing Description support

[k-toumura]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Add documentation for WoT Thing Description support.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage remained the same at 65.161% when pulling 21ab1c9 on node-red-hitachi:wot-docs into 4e97136 on node-red:master.

[k-toumura]

Thank you!