[doc] Add some more description to cores page

Signed-off-by: Marno van der Maas <[email protected]>

Author: marnovandermaas

hw/doc/cores.md

@@ -1,4 +1,9 @@
 # Cores
 
-* [Ibex](../ip/rv_core_ibex/README.md)
-* [OTBN](../ip/otbn/README.md)
+Cores in OpenTitan are processing units that can run programs.
+
+Currently, there are two cores in OpenTitan:
+* [Ibex](../ip/rv_core_ibex/README.md) (RV32IMCB)
+* [OTBN](../ip/otbn/README.md) (programmable coprocessor for asymmetric cryptographic algorithms, 256-bit datapath)
+
+Since cores are the interface between hardware and software, please also consult the [software resources](../../sw/README.md).