added how to add varaible documentation

Author: sbryngelson

doc/how_to_add_variable.md

@@ -0,0 +1,20 @@
+# How to add variable to MFC
+
+* Add variable to input.py
+* Add variable to src/master_scripts/m_python_proxy.py dictionary as appropriate (pre_process, simulation, and/or post_process)
+    * This will then write this variable to the \*.in input file that gets created by input.py
+* Go into the code that you added the variable for (pre_process / simulation / post_process)
+    * m_startup.f90
+      * Add variable to NAMELIST user_inputs in subroutine s_read_input_file
+      * Add appropriate checks to s_check_input_file 
+    * m_global_parameters.f90
+      * Make your new variable/parameter global so that the whole code can see it
+      * Add it to the header of this module (document as appropriate)
+      * In s_assign_default_values_to_user_inputs set the default value for this variable
+    * m_mpi_proxy.f90
+      * s_mpi_bcast_user_inputs
+      * This is where the variable is broadcasted to all processors 
+      * Add to list of broadcasted variables 
+      * e.g. CALL MPI_BCAST(weno_nn, 1, MPI_LOGICAL, 0, MPI_COMM_WORLD, ierr  )
+* You can now use your variable (e.g. weno_nn) in the code. At least anywhere that m_global_parameters is used (see header of module)
+