Update docs re TMG

[mands]

Problem

Following on from nstack/nstack-org#32, need to update docs re usage of both newer and existing features expected to be used by TMG

Tasks

• HTTP API (Add documentation and spec for using the HTTP API #47)
• BigQuery (Take from repo docs - https://github.com/nstack/nstack-std-lib/blob/master/GCP/BigQuery/README.md)
• S3 (Take from repo docs - https://github.com/nstack/nstack-std-lib/blob/master/AWS/S3/README.md)
• Scheduler (Add documentation and specs re Scheduler #49)