imp(docs): add Limitations section #15

Author: omarabid

README.md

@@ -45,6 +45,13 @@ Profile your code:
  // Non-profiled computation
 ```
 
+### Limitations
+
+- **Backend**: The Pyroscope Agent uses [pprof-rs](https://github.com/tikv/pprof-rs) as a backend. As a result, the [limitations](https://github.com/tikv/pprof-rs#why-not-) for pprof-rs also applies.
+- **Tagging**: Adding or removing tags is not possible within threads. In general, the [Pyroscope Agent](https://docs.rs/pyroscope/latest/pyroscope/pyroscope/struct.PyroscopeAgent.html) is not Sync; and as a result a reference cannot be shared between threads. A multi-threaded program could be profiled but the agent is not thread-aware and a particular thread cannot be tagged.
+- **Timer**: epoll (for Linux) and kqueue (for macOS) are required for a more precise timer.
+- **Shutdown**: The Pyroscope Agent might take some time (usually less than 10 seconds) to shutdown properly and drop its threads.
+
 ### Getting help
 
 You can read the [Docs](https://docs.rs/pyroscope/) or check the [examples](examples) for detailed usage of the library. You can also join the [Slack channel](https://pyroscope.slack.com/archives/C02Q47F8LJH) if you have questions.