Add documentation on marker functions / dump file

[loostrum]

• explain use of marker vs just read function
• explain sync with device
• explain where the marker is written (before or after given timestamp?)
• explain parsing of dump file as a whole?

[loostrum]

First two points are addressed in #195
The marker is now written in-line with the sensor values, so point 3 is no longer relevant.
Point 4 is left for later.