[README] revision

The searx project has been archived, differentiation of the project in the
README is no longer necessary.  The README has been reduced to the important
entry points for users, maintainers and developers.

Related: https://github.com/searx/searx/commit/276ffd3f01cdd823f75676c51231fad4040059d3



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>