Add vs00 timed variable string events

[geishm-ansto]

Add a new flatbuffer description that is useful at ANSTO to support logging and variable strings in the nxs file writer.
The read me file was updated and there is no breaking changes.

Approval Criteria

This PR should not be merged until the ECDC Group Leader (acting or permanent) has given their explicit approval in the comments section.
SCIPP/DRAM should also be consulted on changes which may affect them.

[rerpha]

Hi @geishm-ansto , we were thinking of doing something similar at ISIS - nexusformat/definitions#1432 by first adding a new nexus base class (then subsequently a flatbuffers schema here for EPICS string PV value updates) that adds support for strings in the same way NXLog works. I don't know if this fits your use case too?

[geishm-ansto]

Hi @rerpha , it's possible that we could use it but I would need to see the details. At the moment we have implemented vs00 in a local ANSTO variant of the streaming data types and nxs writer but didn't want to add a different python package for reading the data type so raised the PR to see if there was any interest. We wanted to be able to capture system log messages during an experiment using a variable string format.