Add notes about the c api and stacktraces to the guide

guide/src/SUMMARY.md

@@ -8,5 +8,6 @@
 - [Python Class](class.md)
 - [Parallelism](parallelism.md)
 - [Debugging](debugging.md)
+- [Advanced Topics](advanced.md)
 - [Building and Distribution](building-and-distribution.md)
 - [Appendix: Pyo3 and rust-cpython](rust-cpython.md)

guide/src/advanced.md

@@ -0,0 +1,7 @@
+# Advanced topics
+
+## ffi
+
+pyo3 exposes much of python's c api through the `ffi`.
+
+The c api is naturually unsafe and requires you to manage reference counts, errors and specific invariants yourself. Please refer to the [C API Reference Manual](https://docs.python.org/3/c-api/) and [The Rustonomicon](https://doc.rust-lang.org/nightly/nomicon/ffi.html) before using any function from that api.

guide/src/debugging.md

@@ -29,3 +29,12 @@ Activate an environment with the debug interpreter and recompile. If you're on l
 [Download the suppressions file for cpython](https://raw.githubusercontent.com/python/cpython/master/Misc/valgrind-python.supp).
 
 Run valgrind with `valgrind --suppressions=valgrind-python.supp ./my-command --with-options`
+
+## Getting a stacktrace
+
+The best start to investigate a crash such as an segmentation fault is a backtrace.
+
+ * Link against a debug build of python as described in the previous chapter
+ * Run `gdb <my-binary>`
+ * Enter `r` to run
+ * After the crash occured, enter `bt` or `bt full` to print the stacktrace