MMBN3: Adds instructions for using the Legacy Collection ROM for setup (#2120)

Co-authored-by: Silvris <58583688+Silvris@users.noreply.github.com>

worlds/mmbn3/docs/setup_en.md

@@ -12,7 +12,8 @@ As we are using Bizhawk, this guide is only applicable to Windows and Linux syst
   - Windows users must run the prereq installer first, which can also be found at the above link.
 - The built-in Archipelago client, which can be installed [here](https://github.com/ArchipelagoMW/Archipelago/releases)
   (select `MegaMan Battle Network 3 Client` during installation).
-- A US MegaMan Battle Network 3 Blue Rom
+- A US MegaMan Battle Network 3 Blue Rom. If you have the [MegaMan Battle Network Legacy Collection Vol. 1](https://store.steampowered.com/app/1798010/Mega_Man_Battle_Network_Legacy_Collection_Vol_1/)
+on Steam, you can obtain a copy of this ROM from the game's files, see instructions below.
 
 ## Configuring Bizhawk
 
@@ -35,6 +36,14 @@ To do so, we simply have to search any GBA rom we happened to own, right click a
 the list that appears and select the bottom option "Look for another application", then browse to the Bizhawk folder
 and select EmuHawk.exe.
 
+## Extracting a ROM from the Legacy Collection
+
+The Steam version of the Legacy Collection contains unmodified GBA ROMs in its files. You can extract these for use with Archipelago.
+
+1. Open the Legacy Collection Vol. 1's Game Files (Right click on the game in your Library, then open Properties -> Installed Files -> Browse)
+2. Open the file `exe/data/exe3b.dat` in a zip-extracting program such as 7-Zip or WinRAR.
+3. Extract the file `rom_b_e.srl` somewhere and rename it to `Mega Man Battle Network 3 - Blue Version (USA).gba`
+
 ## Configuring your YAML file
 
 ### What is a YAML file and why do I need one?
@@ -76,4 +85,4 @@ Don't forget to start manipulating RNG early by shouting during generation:
 JACK IN!
 [Your name]!   
 EXECUTE!
-```
+```