Add react 17.x to peer dependencies

[kamioftea]

415 Notes that the compatibility for React 17.x.x has been added in 0.16.4, but as this hasn't been reflected in package.json, npm install react-rte still fails in a project using react 17.x.x.

npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR! 
npm ERR! While resolving: campaign-of-kings@undefined
npm ERR! Found: react@17.0.2
npm ERR! node_modules/react
npm ERR!   react@"17.0.2" from the root project
npm ERR! 
npm ERR! Could not resolve dependency:
npm ERR! peer react@"0.14.x || 15.x.x || 16.x.x" from react-rte@0.16.4
npm ERR! node_modules/react-rte
npm ERR!   react-rte@"*" from the root project
npm ERR! 
npm ERR! Fix the upstream dependency conflict, or retry
npm ERR! this command with --force, or --legacy-peer-deps
npm ERR! to accept an incorrect (and potentially broken) dependency resolution.

This adds 17.x.x to the supported peerDependencies for react and react-dom.

Fixes #419

[sstur]

Thanks @kamioftea!