Update usage.md

[imarovec28]

import {
web3Accounts,
web3Enable,
web3FromAddress,
web3ListRpcProviders,
web3UseRpcProvider
} from '@polkadot/extension-dapp';

// returns an array of all the injected sources
// (this needs to be called first, before other requests)
const allInjected = await web3Enable('my cool dapp');

// returns an array of { address, meta: { name, source } }
// meta.source contains the name of the extension that provides this account
const allAccounts = await web3Accounts();

// the address we use to use for signing, as injected
const SENDER = '5DTestUPts3kjeXSTMyerHihn1uwMfLj8vU8sqF7qYrFabHE';

// finds an injector for an address
const injector = await web3FromAddress(SENDER);

// sign and send our transaction - notice here that the address of the account
// (as retrieved injected) is passed through as the param to the signAndSend,
// the API then calls the extension to present to the user and get it signed.
// Once complete, the api sends the tx + signature via the normal process
api.tx.balances
.transfer('5C5555yEXUcmEJ5kkcCMvdZjUo7NGJiQJMS7vZXEeoMhj3VQ', 123456)
.signAndSend(SENDER, { signer: injector.signer }, (status) => { ... });

[jacogr]

We cannot drop all this information, it will make the site break.