Improve SDK readme to include faucet and engagement rewards

[sirpy]

Motivation

Devs should know about these two features

Details

1. add to the main readme links + short description
2. make sure faucet and engagement sdk readmes are clear and up to date
3. remove redundant specific sdk examples from the main readme that already appear in the sdk own readme

Other readme updates

1. include readme section for custodial sdk
2. update readme to include and reflect changes involving the react-hooks update