Add documentation on how to use rust to create sailfish apps

[thigg]

I understand that this is not yet officially supported, yet it would be nice to have some documentation on how to use this, as it would ease experiments with rust on sailfish.

[kunerd]

Here is an example on how to build a rust app for Sailfish in general:
https://github.com/R1tschY/harbour-rust-example

But it would be nice to have some documentation on how to update the rust version that comes with the build system, to experiment with newer versions.