Update Porting from Classic ZGB wiki page

[joshbalfour]

I just successfully ported my game from Classic ZGB to CrossZGB. my only required change was the Makefile,
I don't think you can do PRs for repo wikis but figured it was worth sharing the steps in case you wanted to update the relevant wiki page: https://github.com/gbdk-2020/CrossZGB/wiki/Porting-from-Classic-ZGB

• Replace the Makefile in your src directory with the new makefile and set your desired TARGETS

The diff should look something like this:

@@ -1,14 +1,17 @@
 PROJECT_NAME = ZGB_TEMPLATE
 
-all: build_gb
+# Set platforms to build here, spaced separated. (These are in the separate Makefile.targets)
+# They can also be built/cleaned individually: "make gg" and "make gg-clean"
+# Possible are: gb gbc pocket megaduck sms gg
+TARGETS=gbc pocket
+
+# Builds all targets sequentially
+all: $(TARGETS)
 
 # Number of banks (must be a power of 2): A (Automatic), 2, 4, 8, 16, 32...
 N_BANKS = A
 
-# Music player: HUGETRACKER(default) or GBT_PLAYER
-MUSIC_PLAYER = HUGETRACKER
-
 # Default hardware sprites size: SPRITES_8x16(default) or SPRITES_8x8
 DEFAULT_SPRITES_SIZE = SPRITES_8x16