Write new documentation about android cross compilation

[brson]

For 0.6 I think we will basically be able to cross compile code to Android, and this will be a notable feature in the release announcement. Let's have some docs prepared so people can try it.

Android is still not going to be 'officially' supported, so I think it is most appropriate to keep this documentation on the wiki, under the 'Docs' section for now. We can link to it from the release notes.

There are some out of date docs here.

[yichoi]

I've updated docs here briefly

[sonwow]

I've updated docs here more detailed

[brson]

Thanks!