Update README with Notes on Security

[lmarsden]

Action from Threat Modelling session of 03/06/20.

As some security concerns are known to exist around libgsf (see e.g. https://github.com/schwehr/generic-sensor-format/issues), make clear in the README that it is the responsibility of the calling application to mitigate these where necessary.

Likewise, it should be reiterated to users of gsfpy that GSF data processed by the package should come from reputable sources and should be integrity checked where possible, as it is a possible attack vector.

ACs

• README updated with notes on security including the points mentioned above.