Add license texts

[michel-slm]

Fixes #13

sptr on  add-license [+]
❯ cargo package --allow-dirty --no-verify
   Packaging sptr v0.3.2 (/home/michel/src/github/Gankra/sptr)

sptr on  add-license [+]
❯ tar tf target/package/sptr-0.3.2.crate
sptr-0.3.2/.github/workflows/rust.yml
sptr-0.3.2/.gitignore
sptr-0.3.2/Cargo.toml
sptr-0.3.2/Cargo.toml.orig
sptr-0.3.2/LICENSE-APACHE
sptr-0.3.2/LICENSE-MIT
sptr-0.3.2/README.md
sptr-0.3.2/src/func.rs
sptr-0.3.2/src/int.rs
sptr-0.3.2/src/lib.rs

Signed-off-by: Michel Alexandre Salim [email protected]