updated reference

Build setup links to main readme now.

doc/TMK_README.md

@@ -113,7 +113,7 @@ Third party libraries like LUFA, PJRC and V-USB have their own license respectiv
 
 Build Firmware and Program Controller
 -------------------------------------
-See [doc/BUILD_GUIDE.md](tmk_core/doc/BUILD_GUIDE.md), or the readme in the particular keyboards/* folder.
+See [build environment setup](/readme.md#build-environment-setup), or the readme in the particular keyboards/* folder.