improve documentation on gislinkagefile and shapefiles #5

[vincentvanhees]

There was not clear documentation yet on gislinkagefile and shapefile format. I have now added a basic description here in hbGIS README, but it may be good to include a more elaborate description in https://github.com/habitus-eu/docs.

Fixes #5

Checklist before merging:

• Existing tests still work (check by running the test suite, e.g. from RStudio).
• Added tests (if you added functionality) or fixed existing test (if you fixed a bug).
• Updated or expanded the documentation.
• Updated release notes in inst/NEWS.Rd with a user-readable summary. Please, include references to relevant issues or PR discussions.
• Added your name to the contributors lists in the DESCRIPTION and CITATION.cff files.

[vincentvanhees]

@josefheidler it would be good to get your feedback on this PR. As I describe above I hope this description is sufficient for hbGIS, but in https://github.com/habitus-eu/docs we probably want a more elaborate discussion of all the expect data formats: accelerometer, GPS, gislinkagefile, and shape files.