doc: add libspot-rs install and usage

[shenxiangzhuang]

Close #22

[shenxiangzhuang]

Hi @asiffer , I find render only some sections of a README.md file as talked at #22 is not very convenient, so I write a new one and just use the remote code file only.

[asiffer]

Ok great! No worries for the js build. So do you need url_download in pymdownx.snippets config ?

[shenxiangzhuang]

I think it's necessary if we want to sync the example code, because we need download the simple.rs file from a remote repo. But it can be optional, I can just paste the code in simple.rs to the doc too. Which method do you prefer?

[asiffer]

No ok that's fine!