1. Convert to README.md
2. Sync come content with the updates on source.android.com
3. fs_config uses bionic/libc/kernel/uapi/linux/capability.h, not
system/core/include/private/android_filesystem_capability.h as the
capability header, so update the documentation accordingly.
Test: n/a
Change-Id: I24a084d7a804d3f5d2259cfcea85b8ff4e79d290
