Adding a readme for wdmergers

Exec/science/wdmerger/README.md

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+# Input parameters to simulate WD mergers
+
+### Problem set up
+```
+problem.problem = 1
+```
+### Easiest way to make the two stars merge
+
+```
+problem.roche_radius_factor = 1.0e0
+```
+This is used to calculate the orbital distance and the rotational period. (Ref: https://articles.adsabs.harvard.edu/pdf/1983ApJ...268..368E)
+
+### Desired masses or densities
+
+Castro either uses the initial WD masses or the initial central densities to generate desired stars. Users can specify one of these and leave the other parameter negative. If both are provided, it will use the central densities to generate the model and assume masses as an estimate for the purpose of determining the envelope mass boundary. This is established [here](https://github.com/AMReX-Astro/Castro/blob/10534968cf09ba73f1044186389740559adc4188/Util/model_parser/model_parser.H#L208).
+
+**Primary**
+```
+problem.mass_P = 1.0e0
+problem.central_density_P = -1.0e0
+```
+
+**Secondary**
+```
+problem.mass_S = 1.0e0
+problem.central_density_S = -1.0e0
+```
+
+### Desired compositions
+
+Inspired by Dan et al. 2012, we classify the WDs using their masses and the maximum values that they can hold, and set their compositions accordingly. 
+
+Stars with masses less than 0.45 $M_{\odot}$ are given a pure He composition. This maximum limit can be changed using:
+```
+problem.max_he_wd_mass =  0.45e0
+```
+
+To simulate stars with pure CO cores and a thin He shell between 0.45 $M_{\odot}$ and 0.6 $M_{\odot}$ we use:
+
+```
+problem.max_he_wd_mass =  0.45e0
+problem.max_hybrid_wd_mass = 0.6e0 
+
+problem.hybrid_wd_c_frac = 0.50e0
+problem.hybrid_wd_o_frac = 0.50e0
+problem.hybrid_wd_he_shell_mass = 0.10e0
+```
+On the other hand, to simlute stars with pure CO cores and a He shell between 0.6 $M_{\odot}$ and 1.05 $M_{\odot}$, and with different CO mass fractions, we use:
+```
+problem.max_hybrid_wd_mass = 0.6e0
+problem.max_co_wd_mass = 1.05e0
+
+problem.co_wd_c_frac = 0.40e0
+problem.co_wd_o_frac = 0.60e0
+problem.co_wd_he_shell_mass = 0.0e0
+```
+**NOTE:** To create stars with masses higher than the default maximum values, we need too increase the limits otherwise the conditions will not be satisfied and hence, we won't get the desired compositions. 
+
+### Stellar temperature
+We can set the initial stellar temperature to the desired value using 
+```
+problem.stellar_temp = 1.0e7
+```