Document a lil bit more the package

Author: mofeing

README.md

@@ -1,6 +1,57 @@
 # Extrae
 
-Julia bindings to `extrae` Basic API.
+Julia bindings to BSC's [`extrae`](https://tools.bsc.es/extrae) HPC profiler.
+
+It supports automatic instrumentation (through `LD_PRELOAD` mechanism, DynInst is on the way) of MPI, CUDA and pthreads, and PAPI/PMAPI hardware counters and callstack sampling.
+Generated traces can be viewed with [Paraver](https://tools.bsc.es/paraver).
+
+## Usage
+
+First, you need to set the Extrae configuration using environment variables or XML configuration. An example configuration file can be found in `scripts/extrae.xml`.
+
+### Event emision
+
+Extrae's functionality is very basic: every registered event is just a tuple of 2 integers annotating the event type and the event value.
+Some events are automatically registered, such as MPI call names when you are tracing or PAPI hardware counters when performing sampling.
+But you can also emit your own custom events using `emit`:
+
+```julia
+# emit event 80000 with value 4
+emit(80_000, 4)
+```
+
+Event types are encoded with `Int32` and the values must always be a `Int64`.
+
+If you want to assign a string descriptor to the event, you should call `Extrae.register` before initialization.
+
+```julia
+const BANANAS_TYPECODE::Int32 = 80_000
+Extrae.register(BANANAS_TYPECODE, "Bananas")
+```
+
+Alternatively, you can also add string descriptors to values.
+
+```julia
+Extrae.register(Int32(80_001), "Monkey name", Int64[0,1,2], String["no monkey", "louis", "george"])
+```
+
+### Initialization
+
+`Extrae` can be initialized just by calling `Extrae.init()`. If you are planning to use `Distributed`, you should call
+
+```julia
+@everywhere Extrae.init(Val(:Distributed))
+```
+
+to properly initialize the profiler in all workers. If you plan to use MPI, you should use the `LD_PRELOAD` mechanism.
+
+The profiling is finished with `Extrae.finish()`.
+
+### Mark user functions
+
+Many times, the profiler catches much more information than we want. One way to filter it is by marking which moments in the trace where devoted to user code. This can be done by calling `Extrae.user_function(1)` to start and `Extrae.user_function(0)` to end the marking region.
+
+We also provide a `Extrae.@user_function` for code cleanliness.
 
 ## Example scripts