Document stable core store getters

[warm-coolguy]

The core store offers a variety of getters, some of them only optionally working when extendedMasterportalapiMarkers is configured. While the configuration is documented, the getters are not, and only their concept is noted.

Within this issue, stable core getters are to be documented in the core's README.md.

[Softwarecules]

Done with PR #215