Add build documentation about sevctl

[tylerfanelli]

@surajssd Apologies for delay, addressed in #157

[surajssd]

Normally built binary has dependency on the following system libs:

$ ldd target/release/sevctl
        linux-vdso.so.1 (0x00007ffed59d1000)
        libssl.so.3 => /lib/x86_64-linux-gnu/libssl.so.3 (0x00007f5d6d8c1000)
        libcrypto.so.3 => /lib/x86_64-linux-gnu/libcrypto.so.3 (0x00007f5d6d47e000)
        libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f5d6d45e000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f5d6d236000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f5d6dbea000)

To be able to do static build I had to also install the musl-tools on top of the existing packages:

sudo apt install -y pkg-config libssl-dev asciidoctor musl-tools
rustup target add x86_64-unknown-linux-musl
cargo build --release --target x86_64-unknown-linux-musl

And using the changes in PR #112, I can now do static build:

$ ldd target/x86_64-unknown-linux-musl/release/sevctl
        statically linked