Add hints for initlized tracer after model start

Added instructions for initializing the tracer after the WRF model start time. 
Thanks to Sudheer Bhimireddy from University of Texas San Antonio, he tested the approach commented in the bottom of this script.
You need to add another conditional statement at the beginning IF ( grid%itimestep .GE. tracer_time ) THEN
Where tracer_time is the desired_spin-up_time(in seconds)/time_step(in seconds)

modificaions/solve_em.F.bkb.tracer

@@ -1,3 +1,4 @@
+
 ! Brian Blaylock
 ! 1 February 2016
 
@@ -68,3 +69,16 @@ IF ( config_flags%tracer_opt == 2 ) THEN
 END IF
 !---------------------------------------------------------------------
 !---------------------------------------------------------------------
+
+! Hints for initlizing tracers at time after run begins (thanks to Sudheer Bhimireddy for testing this approach):
+! Use the grid%itimestep variable to add a conditon test whether or not to add a tracer.
+! From his email:
+!! I've created a new namelist variable (tracer_time) under &dynamics, which takes the number of time 
+!! steps as input. I modified the solve_em.F code as:
+!!! tracer_time = desired_spin-up_time(in seconds)/time_step(in seconds) specified in the namelist.input 
+!!! So, for a 6hr spin-up time and a 10sec time step my tracer_time = 2160
+!   IF ( grid%itimestep .GE. tracer_time ) THEN
+!      ! Release the tracers 
+!   END IF
+!! A big limitation is that the timestep has to be constant for this to work. I am writing another routine to make 
+!! it work for adaptive time step.