chore: add debug profile + instructions to readme (#4494)

After troubleshooting my failure to debug Relay using VSCode, I found
that we had `debug` set to `false` in the dev profile. Added a note to
the readme about it in case anyone else runs into the same issue.

---------

Co-authored-by: Joris Bayer <[email protected]>

Author: mjq

Cargo.toml

@@ -8,6 +8,12 @@ resolver = "2"
 # target folder, but we don't require stack traces in most cases.
 debug = false
 
+[profile.dev-debug]
+# A version of the dev profile with debug information enabled, for e.g. local
+# debugging.
+inherits = "dev"
+debug = true
+
 [profile.release]
 # In release, however, we do want full debug information to report
 # panic and error stack traces to Sentry.

README.md

@@ -183,6 +183,12 @@ make format
 make lint
 ```
 
+### Debugging
+
+Developer builds do not include debug information by default. If you want to
+attach a debugger to your local Relay, you can use the provided `dev-debug`
+profile instead with `cargo run --all-features --profile dev-debug`.
+
 ### Python and C-ABI
 
 Potentially, new functionality also needs to be added to the Python package.