internals/memory-debugging.md: add the how to find a leak chapter

internals/memory-debugging.md

@@ -27,3 +27,56 @@ list 'TrackMemory' feature for curl debug memory tracking capable
 builds. These features are independent and can be controlled when running the
 configure script. When `--enable-debug` is given both features will be
 enabled, unless some restriction prevents memory tracking from being used.
+
+## Track Down Memory Leaks
+
+... using the memory debug system. In general, we suggest using valgrind a the
+first choice.
+
+### Single-threaded
+
+  Please note that this memory leak system is not adjusted to work in more
+  than one thread. If you want/need to use it in a multi-threaded app. Please
+  adjust accordingly.
+
+### Build
+
+  Rebuild libcurl with `-DCURLDEBUG` (usually, rerunning configure with
+  `--enable-debug` fixes this). `make clean` first, then `make` so that all
+  files are actually rebuilt properly. It will also make sense to build
+  libcurl with the debug option (usually `-g` to the compiler) so that
+  debugging it will be easier if you actually do find a leak in the library.
+
+  This will create a library that has memory debugging enabled.
+
+### Modify Your Application
+
+  Add a line in your application code:
+
+```c
+  curl_dbg_memdebug("dump");
+```
+
+  This will make the malloc debug system output a full trace of all resources
+  using functions to the given file name. Make sure you rebuild your program
+  and that you link with the same libcurl you built for this purpose as
+  described above.
+
+### Run Your Application
+
+  Run your program as usual. Watch the specified memory trace file grow.
+
+  Make your program exit and use the proper libcurl cleanup functions etc. So
+  that all non-leaks are returned/freed properly.
+
+### Analyze the Flow
+
+  Use the `tests/memanalyze.pl` perl script to analyze the dump file:
+
+    tests/memanalyze.pl dump
+
+  This now outputs a report on what resources that were allocated but never
+  freed etc. This report is fine for posting to the list.
+
+  If this does not produce any output, no leak was detected in libcurl. Then
+  the leak is mostly likely to be in your code.