provide a basic readme with an example on how to eval a result in js

[r-52]

this PR introduces new blocks in the readme to showcase how to eval
a string to a value.

BTW: the minor version is based on the latest published crates.io version. Should this be changed to the latest master? Or is a new release planned?

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This change is 

[jdm]

We are no longer able to publish the mozjs crate (and transitively the rust-mozjs crate) to crates.io because of rust-lang/cargo#6917. The documentation should not recommend using a published version because of this.

[r-52]

@jdm do you think it makes sense to switch from the version-number-scheme to a Git-url? Or should this be removed?

[jhwgh1968]

Hi @romankl, I wish I had seen your PR sooner. I created #489 in December, which does something similar in the crate doc string. The main difference is that mine refers to the online User Guide, and specifically translates two example blocks.

Your code helped me when I was tripping over some things in the Rust API. Could we merge our code somehow?

[jdm]

My fear with adding this code to the readme in this form is that it will potentially get out of date when we upgrade spidermonkey, since it's not automatically compiled and validated in CI. I would rather add an examples directory that we can check with cargo build --all-examples.