update README file

README.md

@@ -1,14 +1,23 @@
+CRETE (Comet RadiativE Transfer and Excitation)
+-------
+
+CRETE is a 1D water excitation and radiation transfer code for
+sub-millimeter wavelengths based on the `RATRAN` code [(Hogerheijde & van der Tak
+2000)](http://adsabs.harvard.edu/abs/2000A%26A...362..697H).
+The code considers rotational transitions of water molecules given
+a Haser spherically symmetric distribution.
+
 Installing the code
 -------
 
 From the top-level directory, run the `configure.sh` shell script to install
-`rat4com`.
+`crete`.
 
 ```
 ./configure.sh
 ```
 
-This script is a simplified version of the original `ratran`'s `configure`
+This script is a simplified version of the original `RATRAN`'s `configure`
 script that works for `bash` and `zsh` shells on Linux, and probably also
 MacOS, using the `gfortran` compiler.
 
@@ -21,10 +30,25 @@ you can instead run the csh script to install the program:
 
 After running either of these scripts you can add the definition of the RATRAN
 and RATRANRUN environment variables to initialize these variables for login
-shells in the shell-specific files `~/.bashrc`, `~/.zshrc` or `~/.cshrc`.
+shells in the shell-specific files `~/.bashrc` or `~/.zshrc`:
 
+```
+export RATRAN=/path/to/crete
+export PATH=$PATH:$RATRAN/bin
+export RATRANRUN=$RATRAN/run
+```
+
+For `~/.cshrc`:
+
+```
+setenv RATRAN /path/to/crete
+set path = ($RATRAN/bin $path)
+setenv RATRANRUN $RATRAN/run
+```
 
 The IR radiation is specified as pumping coefficients in the file
 `amc/getmatrix.f`.  One needs to use the `g_ir` array for ortho- or
-para-water by uncommenting the corresponding definition and include
-the heliocentric distance to scale the coefficients in lines 103 and 104.
+para-water by uncommenting the corresponding definition from [Zakharov et al
+2007](http://adsabs.harvard.edu/abs/2007A%26A...473..303Z). The model file
+includes the heliocentric distance of the source to scale the IR pumping
+coefficients.