Add LICENSE file

[calejvaldez]

I understand that the license is specified in the Cargo.toml file, but I think it'd be nice for a LICENSE file to exist. It helps to quickly know where to find the license, as well as have GitHub auto-detect the license.

I used the same license I found in the Cargo.toml file:

https://github.com/neil-lobracco/twilio-rs/blob/370f602fa1d1f39d4e242b9a494937f0499566d4/Cargo.toml#L7