[fidl][docs] More details for fidlgen

- regenerate goldens
- notes on fidlgen_dart

Change-Id: Ia6c88cf2b563238faa1ea75e2e29e4fdc86273cd
diff --git a/docs/development/languages/fidl/CONTRIBUTING.md b/docs/development/languages/fidl/CONTRIBUTING.md
index a8beab8..3c0a495 100644
--- a/docs/development/languages/fidl/CONTRIBUTING.md
+++ b/docs/development/languages/fidl/CONTRIBUTING.md
@@ -87,7 +87,7 @@
 ./scripts/run-zircon-x64 -c zircon.autorun.boot=/boot/test/sys/fidl-coding-tables-test
 ```
 
-### fidlgen
+### fidlgen (LLCPP, HLCPP, Rust, Go)
 
 Some example tests you can run:
 
@@ -96,6 +96,26 @@
 fx run-host-tests fidlgen_golang_ir_test
 ```
 
+To regenerate the goldens:
+
+```sh
+fx exec garnet/go/src/fidl/compiler/backend/typestest/regen.sh
+```
+
+### fidlgen_dart
+
+Some example tests you can run:
+
+```sh
+fx run-host-tests fidlgen_dart_backend_ir_test
+```
+
+To regenerate the goldens:
+
+```sh
+fx exec topaz/bin/fidlgen_dart/regen.sh
+```
+
 ### C runtime
 
 In `zircon/`: