Need docs for directive resolvers

[stubailo]

#518 by @giautm added a new feature that's shipping in 2.13.0, we need docs for it though! I propose a new page called "Schema directives" to document this stuff.

• There are some docs in the PR description here: Need docs for directive resolvers #530
• Also, some use cases in the original issue: makeExecutableSchema should permit directives #212
• Also, examples can be taken from the tests: https://github.com/apollographql/graphql-tools/blob/cc69bb1556893c9828448470036d02932e3b01a8/src/test/testSchemaGenerator.ts#L2024

To run the docs:

1. Go to /docs in the repo
2. git submodule init && git submodule update
3. npm install
4. npm start

Adding a page can be done by:

1. Adding a file in the docs/source directory
2. Adding an entry with the filename to docs/_config.yml
3. Don't forget to put the title/description like so: https://raw.githubusercontent.com/apollographql/graphql-tools/master/docs/source/mocking.md

[vladshcherbin]

This one can be also solved with directives and is a good example of this great feature.

[stubailo]

Yeah it can definitely be a good approach to authorization!

[stubailo]

@giautm neat!! want to send that as a PR? If not I'll get around to copy-pasting it next week

[DavidHe1127]

@giautm Thanks for your example code. There is a typo in your code.

Wrong

attachDirectiveResolvers(
  schema,
  directiveResolvers,
});

Should be

attachDirectiveResolvers(schema, directiveResolvers);

[Altiano]

Can directive has parameters?
If so, please give me code example

[giautm]

@Altiano: I opened a PR for the document. Please check it, and you can check tests for more examples.