Supplement the examples and documents of ipfs-api-backend-actix crates

[Jzow]

In order to help developers use the new crates, some relevant documents and examples should be provided

[ferristseng]

Thanks! You should actually be able to run any of the examples with Actix (see https://github.com/ferristseng/rust-ipfs-api/blob/master/ipfs-api-examples/src/lib.rs), so I'm going to close this

$ cd ipfs-api-examples
$ cargo run --example add_file --no-default-features --features with-actix