Introduce JS clients in the docs

[ipopescu]

We have examples for JS clients using the erc20 and nft contracts here:

• Main readme: https://github.com/casper-network/casper-contracts-js-clients/
• Code for ERC-20 client: https://github.com/casper-network/casper-contracts-js-clients/tree/master/packages/erc20-client
• Code for NFT client: https://github.com/casper-network/casper-contracts-js-clients/blob/master/packages/cep47-client/README.md

For this task, it would be great to add a small summary to introduce this repository in the documentation. One suggestion is the page talking about the JS SDK: https://casper.network/docs/dapp-dev-guide/sdk/script-sdk.

Note: We don't want to duplicate the readme files, but provide a high-level summary to let people know that these clients exist when they search the docs.