added README-vis.md for documentation on using event tracing

README-vis.md

+## README for using ROSS instrumentation in CODES
+
+For details about the ROSS instrumentation, see the README-vis.md file in the [ROSS repo](https://ross.cs.rpi.edu).
+The instrumentation is still underdevelopment and the vis2 branch must be checked out to use it. 
+
+There are currently 3 types of instrumentation: GVT-based, real time, and event tracing.  See the ROSS documentation for more info on
+the specific options or use `--help` with your model.  The GVT-based and real time sampling do not require any changes to your model code.
+The event tracing does in order to get specific model event types.  This document describes how to do it.
+
+### Register LP event tracing function
+
+The examples here are based on the server LP for the synthetic workload generation for dragonfly (src/network-workloads/model-net-synthetic.c).
+
+As described in the ROSS Vis documentation, we need to first add our function that will save the event type (and any other desired data) to the
+buffer location provided by ROSS.
+```C
+void svr_event_collect(svr_msg *m, tw_lp *lp, char *buffer)
+{
+    int type = (int) m->svr_event_type;
+    memcpy(buffer, &type, sizeof(type));
+}
+```
+
+Then we need to create a `st_trace_type` struct with the pointer and size information.
+```C
+st_trace_type svr_trace_types[] = {
+    {(rbev_trace_f) svr_event_collect,
+    sizeof(int),
+    (ev_trace_f) svr_event_collect,
+    sizeof(int)},
+    {0}
+}
+```
+
+And a function to return this struct
+```C
+static const st_trace_type *svr_get_trace_types(void)
+{
+    return(&svr_trace_types[0]);
+}
+```
+
+As a reminder, there are two types of event tracing the full event trace (`ev_trace_f`) or only events that trigger rollbacks (`rbev_trace_f`).  
+It's set up so that you can have different functions for both types of event tracing, or you can use the same function for both.
+Immediately after each function pointer is a `size_t` type that takes the amount of data that the function will be placing in the buffer,
+so ROSS can appropriately handle things.
+
+If you have multiple LPs, you can do a `st_trace_type` for each LP, or you can reuse.  *Note*: You can only reuse `st_trace_type` and the event type collection
+function for LPs that use the same type of message struct.  For example, the dragonfly terminal and router LPs both use the `terminal_message` struct, so they can
+use the same functions for event tracing.  However the model net base LP uses the `model_net_wrap_msg` struct, so it gets its own event collection function and 
+`st_trace_type` struct, in order to read the event type correctly from the model.  
+
+`codes_mapping_init()` was changed to register the function pointers when it is setting up the LP types.  So for CODES models, you need to add a register function:
+```C
+void svr_register_trace()
+{
+    trace_type_register("server", svr_get_trace_types());
+}
+```
+
+`trace_type_register(const char* name, const st_trace_type* type)` is part of the API and lets CODES know the pointers for LP initialization.
+
+Now in the main function, you call the register function *before* calling `codes_mapping_setup()`.
+```C
+if (g_st_ev_trace)
+    svr_register_trace();
+```
+
+`g_st_ev_trace` is a ROSS flag for determining if event tracing is turned on.  
+
+That's all you need to add for each LP.  
+
+### Model Net LPs
+In addition to the dragonfly synthetic server LP, I've already added in the necessary changes for both the model net base LP type and dragonfly (both router and terminal LPs),
+so no other changes need to be made to those LPs.  
+For any other network LPs that are based on the model net base LP type, there are a few additional details to know.
+There are two fields added to the `model_net_method` struct for pointers to the trace registration functions for each LP.
+
+```C
+void (*mn_trace_register)(st_trace_type *base_type);
+const st_trace_type* (*mn_get_trace_type)();
+```
+
+For example, right now, both the dragonfly router and terminal LPs use the same `st_trace_type dragonfly_trace_types` struct and the following function to return its pointer:
+```C
+static const st_trace_type *dragonfly_get_trace_types(void)
+{
+    return(&dragonfly_trace_types[0]);
+}
+```
+
+They have different register functions:
+```C
+static void dragonfly_register_trace(st_trace_type *base_type)
+{
+    trace_type_register(LP_CONFIG_NM_TERM, base_type);
+}
+
+static void router_register_trace(st_trace_type *base_type)
+{
+    trace_type_register(LP_CONFIG_NM_ROUT, base_type);
+}
+```
+
+And then the following additions to their `model_net_method` structs:
+```C
+struct model_net_method dragonfly_method =
+{
+    // the fields already in the struct
+    ...
+    // event tracing additions
+    .mn_trace_register = dragonfly_register_trace,
+    .mn_get_trace_type = dragonfly_get_trace_types,
+};
+
+struct model_net_method dragonfly_router_method =
+{
+    // the fields already in the struct
+    ...
+    // event tracing additions
+    .mn_trace_register = router_register_trace,
+    .mn_get_trace_type = dragonfly_get_trace_types,
+};
+```
+
+Any other LPs built off of the model net LP, can be changed in the same way.