chore: add CONTRIBUTING.md with release instructions

Author: GuillaumeLagrange

CONTRIBUTING.md

@@ -0,0 +1,7 @@
+# Codspeed C++
+
+## Release
+
+For now, releasing is done manually, by tagging a git rev with vX.Y.Z.
+
+When releasing a new version, please make sure the [version](https://github.com/CodSpeedHQ/codspeed-cpp/blob/fix-walltime-pid/core/CMakeLists.txt#L3) is updated in the `core/CMakeLists.txt` file to match the pushed version