Document non trivial custom mutation

[eric-burel]

Example of a non trivial call gracefully provided by @yairtal

Creating a custom server mutation for any use case that is not provided by Vulcan's default mutation is a perfectly normal pattern, yet it's not a reflex because the syntax is not immediate.
Doc could be improved with a non trivial example + a more visible/higher up section about custom mutations and resolvers.

import { addGraphQLMutation, addGraphQLResolvers, Utils } from 'meteor/vulcan:core';
import Users from 'meteor/vulcan:users';
const check = (user, document) => {
  if (!user || !document) return false;
  return Users.owns(user, document)
    ? Users.canDo(user, 'post.update.own')
    : Users.canDo(user, `post.update.all`);
};
const specificResolvers = {
  Mutation: {
    addToPostsViewedByIds(root, {postId, userId}, context) {
      const document = context.Posts.findOne({_id: postId});
      Utils.performCheck(check, context.currentUser, document);
      context.Posts.update({_id: postId}, {$addToSet: {viewedByIds: userId}});
      return document;
    },
  },
};
addGraphQLResolvers(specificResolvers);
addGraphQLMutation('addToPostsViewedByIds(postId: String, userId: String) : Post');