Add readme with simple instructions

README.md

+# xtrace-cpp
+Barebones C++ implementation of X-Trace compatible with latest brown tracing framework + baggage version
+
+# Building
+
+```
+mkdir build
+cd build
+cmake ..
+make
+sudo make install
+```
+
+# Creating Traces
+
+To start an X-Trace Trace:
+
+```
+#include "xtrace.h"
+
+XTrace::StartTrace();
+```
+
+To log something with X-Trace:
+
+```
+#include "xtrace.h"
+
+XTRACE("hello world! I am an X-Trace event");
+```
+
+Or alternatively
+
+```
+XTrace::log("hello world! I am an X-Trace event");
+```
+
+
+# X-Trace reports server
+
+This implementation sends reports to the `xtrace-server` hostname, on the default X-Trace port of 5563.  These are specified in `pubsub.h`.
+
+TODO: instructions on running report server
+
+# Context Propagation
+
+X-Trace uses thread-local storage to automatically link events together in the same thread.  If you want to log things from a new thread, you have to manually pass this context to a new thread.
+
+```
+#include "thread_local_baggage.h"
+
+Baggage branched_baggage = ThreadLocalBaggage::Branch();
+
+std::thread branched_thread([&branched_baggage]() {
+  ThreadLocalBaggage::Set(branched_baggage);	
+  XTRACE("hello from a new thread!");
+});
+```
+
+To include baggage in an RPC, serialize it as follows:
+
+```
+#include "thread_local_baggage.h""
+
+Baggage baggage_for_rpc = ThreadLocalBaggage::Branch();
+
+std::vector<uint8_t> serialized_bytes = baggage_for_rpc.serialize();
+```
+
+To instantiate baggage received over RPC, deserialize as follows:
+
+```
+#include "atomlayer.h"
+
+std::vector<uint8_t> serialized_bytes;
+
+Baggage received_baggage = Baggage::deserialize(serialized_bytes);
+
+ThreadLocalBaggage::Set(received_baggage);
+```
+
+You can combine multiple baggages into one
+
+```
+#include "atomlayer.h"
+
+Baggage a, b;
+
+Baggage joined = Baggage::merge(a, b);
+
+```
+
+Equivalently
+
+```
+#include "thread_local_baggage.h"
+
+Baggage a, b;
+
+ThreadLocalBaggage::Set(a);
+ThreadLocalBaggage::Join(b);
+```
+
+The file `main.cc` gives some simple examples of X-Trace usage and context propagation
+
+# Read more
+
+Our paper on Eurosys gives more on baggage: [Universal Context Propagation for Distributed System Instrumentation](https://people.mpi-sws.org/~jcmace/papers/mace2018universal.pdf) Jonathan Mace and Rodrigo Fonseca, Eurosys 2018
+
+See also http://github.com/tracingplane
+