Replace README text regarding relayer when it's ready

[mikedotexe]

This section was recently excised in this PR #3 :

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Set up trusted relayer

Visit the simple relayer repository and follow directions to run it. This will run a small ExpressJS server locally at port 3000.

Start web app

env NEAR_RELAY_URL=http:127.0.0.1:3000 npm run betanet

This will run a local website. The terminal will display the URL, which is typically:
http://localhost:1234

Once the relayer mentioned gets in better shape let's remember to add this back.