README update - Contributing & dual license

[elpiel]

• We need dual-license the nmea code with MIT due to the code integration from the yanp crate (yanp integration is no longer needed due to the amount of work to refactor the code without the added benefit of having additional tests) Dual license with MIT and Apache-2.0 #52

• Add Contributing section to README with suggestions on how people can contribute to this crate and be made available in crates.io as well (suggested by @zeenix https://twitter.com/zeenix/status/1567582371297345539, thanks for the suggestions!) fc93169