add option for building cores

README.md

@@ -87,6 +87,12 @@ To start compilation from the beginning, run the following prior to compiling:
 make clean
 ```
 
+#### Building specific cores  
+If you would like to build a subset of cores, you can specify the `cores="core1 core2"` option. Currently available cores:  
+```
+a26 fc sfc n64 sg ms md ps1 pce ng msx cv gb gba ws ngp spec
+```  
+
 Build Output
 ------------