Makefile: 根目录下添加make help命令 (#271)

* Makefile: 根目录下添加make help命令

* Makefile: 补充根目录Makefile的help命令

Makefile

@@ -131,3 +131,21 @@ run-docker:
 	sudo bash tools/build_in_docker.sh || exit 1
 	$(MAKE) write_diskimage || exit 1
 	$(MAKE) qemu
+
+help:
+	@echo "编译:"
+	@echo "  make all -j <n>       - 本地编译，不运行,n为要用于编译的CPU核心数"
+	@echo "  make build            - 本地编译，并写入磁盘镜像"
+	@echo "  make docker           - Docker编译，并写入磁盘镜像"
+	@echo ""
+	@echo "编译并运行:"
+	@echo "  make run-docker       - Docker编译，写入磁盘镜像，并在QEMU中运行"
+	@echo "  make run              - 本地编译，写入磁盘镜像，并在QEMU中运行"
+	@echo "  make run-uefi         - 以uefi方式启动运行"
+	@echo ""
+	@echo "运行:"
+	@echo "  make qemu             - 不编译，直接从已有的磁盘镜像启动运行"	
+	@echo "  make qemu-uefi        - 不编译，直接从已有的磁盘镜像以UEFI启动运行"	
+	@echo ""
+	@echo ""
+	@echo "注: 对于上述的run, run-uefi, qemu, qemu-uefi命令可以在命令后加上-vnc后缀,来通过vnc连接到DragonOS, 默认会在5900端口运行vnc服务器。如：make run-vnc "