Auto merge of #14187 - emilio:rr-docs, r=jdm

Add a few minimal debugging docs.

r? @jdm (or anyone else)

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+# Servo debugging guide
+
+There are a few ways to debug Servo. `mach` supports a `--debug` flag that
+searches a suitable debugger for you and runs servo with the appropriate
+arguments under it:
+
+```
+./mach run --debug test.html
+```
+
+You can also specify an alternative debugger using the `--debugger` flag:
+
+```
+./mach run --debugger=my-debugger test.html
+```
+
+You can also, of course, run directly your debugger on the Servo binary:
+
+```
+$ gdb --args ./target/debug/servo test.html
+```
+
+## Debugging SpiderMonkey.
+
+You can build Servo with a debug version of SpiderMonkey passing the
+`--debug-mozjs` flag to `./mach build`.
+
+Note that this sometimes can cause problems when an existing build exists, so
+you might have to delete the `mozjs` build directory, or run `./mach clean`
+before your first `--debug-mozjs` build.
+
+## Debugging Servo with [rr][rr].
+
+To record a trace under rr you can either use:
+
+```
+$ ./mach run --debugger=rr testcase.html
+```
+
+Or:
+
+```
+$ rr record ./target/debug/servo testcase.html
+```
+
+### Running WPT tests under rr's chaos mode.
+
+Matt added a mode to Servo's testing commands to record traces of Servo running
+a test or set of tests until the result is unexpected.
+
+To use this, you can pass the `--chaos` argument to `mach test-wpt`:
+
+```
+$ ./mach test-wpt --chaos path/to/test
+```
+
+Note that for this to work you need to have `rr` in your `PATH`.
+
+Also, note that this might generate a lot of traces, so you might want to delete
+them when you're done. They're under `$HOME/.local/share/rr`.
+
+### Known gotchas
+
+If you use a Haswell processor that supports Hardware Lock Ellision, rr might
+not work for you. There's a `rr` [bug][rr-bug] open about this. Until that gets
+fixed, you can ensure that the `parking_lot` dependency isn't built with the
+`nightly` feature, which as of this writing is the only dependency that uses it.
+
+[rr]: http://rr-project.org/
+[rr-bug]: https://github.com/mozilla/rr/issues/1883