add contributing guidelines

CONTRIBUTING.md

@@ -0,0 +1,16 @@
+Contributing to dd-trace-cpp
+============================
+Pull requests for bug fixes are welcome.
+
+Before submitting new features or changes to current functionality, [open an
+issue](https://github.com/DataDog/dd-trace-cpp/issues/new) and discuss your
+ideas or propose the changes you wish to make. After a resolution is reached, a
+PR can be submitted for review.
+
+Code Style
+----------
+C++ code is formatted using `clang-format-14`. Before submitting code changes,
+run the following command:
+```shell 
+$ bin/format
+```

README.md

@@ -82,7 +82,6 @@ options is not needed. The `-ldd_trace_cpp` option is always needed.
 ```console
 $ c++ -o my_app my_app.o -L/path/to/dd-trace-cpp/.install/lib -ldd_trace_cpp
 ```
-<!-- TODO: Do those commands need -pthread as well? -->
 
 Test
 ----
@@ -101,6 +100,6 @@ Alternatively, [bin/test](bin/test) is provided for convenience.
 
 Contributing
 ------------
-TODO: pull requests, issues, bin/format
+See the [contributing guidelines](CONTRIBUTING.md).
 
 [1]: https://cmake.org/