Document timestamps manipulations

[danielsf]

We've learned a lot about different ways to handle timestamps over the course of the VBN release. We should record that in a document somewhere. Relevant tickets are

#2370
#2337
#2357
#2383

Tasks

• Draw up a document (a readme page in GitHub) documenting the different ways that sessions returned by the SDK get their timestamps from raw data

Validation

• Approval of document by at least one other member of team Pika