Document xctrace usage (MystenLabs#3062)

Co-authored-by: Mark Logan <[email protected]>

doc/src/contribute/xcode-instruments.md

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+---
+title: Profiling Sui on Mac OS using XCode instruments.
+---
+
+To profile on Mac OS:
+
+* Install XCode: https://apps.apple.com/us/app/xcode/id497799835?mt=12
+* Make sure you have the command line tools but running `xcode-select install`. If it says they are already installed you may want to go directly to https://developer.apple.com/download/more/ and download the .pkg for the commandline tools to make sure they are up to date.
+* Add `/Applications/Xcode.app/Contents/Developer/usr/bin` to your `$PATH`.
+* Build whatever Sui component you want to run normally with `cargo build` - these docs assume `target/debug/sui`.
+* Sign the binary you wish to profile:
+    * Create a file called `debug.plist` with the following contents:
+
+                <?xml version="1.0" encoding="UTF-8"?><!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "https://www.apple.com/DTDs/PropertyList-1.0.dtd"><plist version="1.0"><dict><key>com.apple.security.get-task-allow</key><true/></dict></plist>
+
+    * Run:
+            $ codesign -s - -v -f --entitlements ../debug.plist target/debug/sui
+* Now run the app and record a trace (select the most appropriate template, see xcode documentation for available templates).
+
+        $ xcrun xctrace record --template 'Allocations' --launch -- ./target/debug/sui start
+
+* Ctrl-C the app when you've recorded enough.
+* It will write a directory starting with `Launch_` - run:
+
+        $ open Launch_xxxxxxxx
+
+* The trace should open in XCode.