Update cprnc README with configure changes.

tools/cprnc/README

@@ -11,10 +11,19 @@ Quick Start Guide:
 On cime supported systems you can generate a Macros file using the following
 (assuming you are running the command from the directory cime/tools/cprnc):
 
-../configure -cimeroot ../../ -mach $machinename -compiler $compilername
+CIMEROOT=../.. ../configure --macros-format=Makefile
+
+To change the compiler or MPI library used, or to enable debugging options,
+set the COMPILER, MPILIB, or DEBUG environment variables prior to running
+configure.
+
+Next, run make to build cprnc. For instance, using sh/bash as a login shell:
+
+CIMEROOT=../.. source .env_mach_specific.sh && make
+
+Finally, put the resulting executable in CCSM_CPRNC as defined in
+config_machines.xml.
 
-Then run make to build cprnc and put the resulting executable in
-CCSM_CPRNC as defined in config_machines.xml
 You can also build cprnc using cmake.
 
  Usage: cprnc  [-v] [-d dimname:start[:count]] file1 [file2]