Improve YARP docs

Author: eregon

doc/contributor/yarp.md

@@ -1,4 +1,16 @@
-# Update YARP
+# YARP
+
+## Update YARP
 
 * Clone `yarp` as a sibling of `truffleruby-ws`.
 * Run `tool/import-yarp.sh` in the truffleruby repo.
+* Commit the result with message `Import Shopify/yarp@COMMIT`
+
+## Print Detailed YARP AST
+
+```bash
+cd yarp
+chruby 3.2.
+bundle exec rake
+bundle exec ruby -Ilib -ryarp -e 'pp YARP.parse("1&.itself")'
+```