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) #52

• [x] 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!) https://github.com/AeroRust/nmea/commit/fc93169a8eca406e165d5fd9ef8593378ded1e83