update README

README.md

@@ -26,6 +26,7 @@ $ brew install FileSottile/musl-cross/musl-cross
 
 ```bash
 $ make {ucore,biscuit,rust,all} arch={i386,x86_64,riscv32,riscv64,aarch64}
+$ make sfsimg arch={i386,x86_64,riscv32,riscv64,aarch64}
 ```
 
 Output to `build/$(arch)`

build-gcc-musl.md

@@ -1,3 +1,17 @@
+#method 1
+## install steps
+```
+wget https://www.musl-libc.org/releases/musl-1.1.21.tar.gz
+tar zxf musl-1.1.21.tar.gz
+cd musl-1.1.21/
+./configure --prefix=/home/rcore/env/musl-install
+make && make install
+cd /home/rcore/env/musl-install/bin
+ln -s musl-gcc x86_64-linux-musl-gcc
+```
+
+
+# method 2
 ## step 1: build musl-gcc with gcc-6.4+musl-1.1.21
 ```
 git clone https://github.com/richfelker/musl-cross-make.git