Update document: README.md

[wxiaoguang]

1. Add a hint for using make TARGET=i486-linux-musl.
2. Add a hint for installing gcc and g++ (I encounter errors when there is no g++ installed)
3. Remove duplicate mips-linux-musl: because just like arm[eb]-linux-musleabi[hf], mips[el]-linux-musl[sf] should be enough.

Thank you for the great project!