Add write example to README

[wtchen]

There's a small example on docs.rs, but it's a bit barebones and doesn't cover a potential use case. We should add a more useful example, and feature it on the README. For example, construct a new Gpx object from scratch and save it to a file.

[lnicola]

That sounds good, would you like to file a PR?

[wtchen]

Yes, I will do it this weekend :-)

[wtchen]

Ok, created the PR