DExx_.._Cramps: add local config readme

Signed-off-by: Michael Brown <producer@holotronic.dk>

Author: the-snowwhite

HW/hm2/config/DExx_Nano_xxx_Cramps/readme.md

@@ -0,0 +1,10 @@
+Shared configs for the DE0_Nano_SoC_Cramps and DE10_Nano_FB_Cramps projects
+The DExx ..._Cramps projects are minimalistic as only the hm2 cores in current configs are included.
+
+A new config is added by creating and then editing a new copy of these 2 files.
+
+    PIN_(your-custom-name).vhd
+    atlas_(your-custom-name).sv
+
+If you wish to create a new config including a not yet (in current configs) included core,
+post a DExx_.._Cramps new core feature request. (in the GGroup).