Add readme

Create a readme file describing the standard distribution.

The README in core should then be changed to only contain relavant information about the core, plus a reference to the standard distribution.

created merge request !5 (merged) to address this issue

mentioned in merge request !5 (merged)

mentioned in commit 306ebe42

mentioned in commit AlekSIS-Core@f253f2cc

mentioned in merge request AlekSIS-Core!406 (merged)

assigned to @debdolph

mentioned in commit 2cd6815a

mentioned in commit 4c4faf06

mentioned in commit AlekSIS-Core@ccf0d6e6

closed with merge request !5 (merged)

mentioned in commit bb651e6a

closed with commit 4c4faf06

mentioned in commit AlekSIS-Core@a862c865