update README

[marktani]

• it doesn't contain information on getting started (install graphql-cli/graphcool)
• it should list a few example queries and mutations

Currently, it's difficult to know what do to.

A common error I saw is running writePost as your first operation, and mistakenly use a JWT that was signed with GRAPHCOOL_SECRET instead of the JWT_SECRET. In this case, the error message

{
  "data": null,
  "errors": [
    {
      "message": "invalid signature",
      "locations": [
        {
          "line": 1,
          "column": 10
        }
      ],
      "path": [
        "writePost"
      ]
    }
  ]
}

is not helpful.