Directives at the top of a SDL file

In the latest [Working Group meeting](https://github.com/graphql/graphql-wg/blob/master/notes/2018-02-01.md) Graphcool presented [GraphQL Import](https://github.com/graphcool/graphql-import). It was brought up that we could change the syntax to be based on directives instead of comments:

```graphql
@import(defs: [Post, Comment] from: "./other-types.graphql")
```

instead of

```graphql
# import Post, Comment from "./other-types.graphql"
```

In the current spec, directives must always be placed after another node. As such, there is no way to place a directive at the top of a file. I volunteered to create an RFC to rectify this using the `@import` directive as motivation.

Discussion in the issue for [graphql-import](https://github.com/graphcool/graphql-import/issues/89#issuecomment-368077528) let to the conclusion that import should not switch to a directive based syntax.

I still think it might be valuable to allow directives in the beginning of a file and would love to hear other use cases for this before proceeding with an RFC.