Update README.md

Despite many readings, I missed the two links to core.cljx and core_test.cljx.  Doh!

I think the links would be more obvious (and be more readily recognized in importance) with some mods. 
1. Remove the "source code" formatting
2. Modify the text to more clearly indicate these are 2 source code files
3. Replace the namespace name with the filename to emphasize #2

Does this look OK?
Alan