Put how to use the specified compiler into the README, and remove it from the makefile

Makefile

@@ -3,7 +3,7 @@ DOTNET = xxdk.NET
 .PHONY: all windows-x64 windows-arm64 linux-x64 linux-arm64 darwin-x64 darwin-arm64 dotnet
 
 libxxdk-win-x64.dll:
-	CGO_ENABLED=1 GOOS=windows CC=x86_64-w64-mingw32-gcc GOARCH=amd64 go build -buildmode=c-shared -o $@ ./sharedcgo
+	CGO_ENABLED=1 GOOS=windows GOARCH=amd64 go build -buildmode=c-shared -o $@ ./sharedcgo
 windows-x64: libxxdk-win-x64.dll
 	mkdir -p $(DOTNET)/runtimes/win-x64/native
 	cp *.h $(DOTNET)/runtimes/win-x64/native/

README.md

@@ -14,6 +14,15 @@ cd xxdk.NET
 dotnet run --ndf mainnet-ndf.json --state-dir world --wait 20 | grep ^DM
 ```
 
+NOTE: you may need to specify a compiler to the make command, especially
+when cross compiling. Example for compiling to windows:
+```
+CC=x86_64-w64-mingw32-gcc make -k
+cd xxdk.NET
+dotnet run --ndf mainnet-ndf.json --state-dir world --wait 20 | grep ^DM
+```
+
+
 It's highly recommended to run with grep on the output. Especially on
 mainnet, logs can be noisy with failure to connect errors as it accesses
 the network.