Adding README file for the CODES mapping API (replicated on the codes webpage)

src/util/README.codes-mapping.txt

+- The codes-mapping.h header file contains the function definitions for the codes LP mapping.
+
+Step 1: Specifying the LP types in the config file:
+
+- Here is an example of a config file that specifies the codes LP mapping
+-------------------------------example-test.conf-----------------------------
+LPGROUPS
+{
+   MODELNET_GRP
+   {
+      repetitions="16";
+      server="1";
+      example_net="1";
+   }
+}
+------------------------------------------------------------------------------
+In this config file, there are multiple LP types defined in a single LP group (As we will see in a later
+example, there can be multiple LP groups in a config file too). There is 1 server LP and 1 example_net
+LP type in a group and this combination is repeated 16 time (repetitions="16"). ROSS will assign the 
+LPs to the PEs (PEs is an abstraction for MPI rank in ROSS) by placing 1 server LP then 1 example_net
+LP a total of 16 times. This configuration is useful if there is some form of communication involved
+between the server and example_net LP types, in which case ROSS will place them on the same PE and 
+communication between server and example_net LPs will not involve remote messages. 
+
+The number of server and example_net LPs can be more than 1. Lets assume if we have two example_net
+LPs for each server then the config file will have the following format:
+
+-------------------------------example-test2.conf-----------------------------
+LPGROUPS
+{
+   MODELNET_GRP
+   {
+      repetitions="16";
+      server="1";
+      example_net="2";
+   }
+}
+------------------------------------------------------------------------------
+
+Step 2: Loading the config file in the model:
+
+After the initialization function calls of ROSS (tw_init), the configuration file can be loaded in the
+example program using:
+
+configuration_load(example-test.conf, MPI_COMM_WORLD, &config);
+
+Step 3: Each LP type must register itself  with the lp_type_register before setting up the codes-mapping. 
+Following is an example function that registers 'server' lp type.
+
+static void svr_add_lp_type()
+{
+  lp_type_register("server", svr_get_lp_type());
+}
+
+Step 4: Call to codes_mapping_setup that sets up the LPs of each registered LP type as specified in the config
+file.
+
+codes_mapping_setup();
+
+Step 5: Querying number of LPs in a repetition
+
+int codes_mapping_get_lp_count(group_name, lp_type_name);
+
+For example, to query the number of server LPs in example-test2.conf file, calling
+
+num_servers = codes_mapping_get_lp_count("MODELNET_GRP", "server");
+ 
+will return 2.
+
+Step 6: Querying number of repetitions in a particular group, use
+
+int codes_mapping_get_group_reps(group_name);
+
+For example, to query the number of repetitions in example-test2.conf file, calling
+
+num_repetitions = codes_mapping_get_group_reps("MODELNET_GRP");
+
+will return 16.