add graphql layer with lang#graphql layer (#2287)

* add graphql layer to docs

* add graphql layer to add graphql file support

autoload/SpaceVim/layers/lang/graphql.vim

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+"=============================================================================
+" graphqlfile.vim --- layer for editing graphql files
+" Copyright (c) 2018 Pieter Joost van de Sande & Contributors
+" Author: Pieter Joost van de Sande < pj at born2code.net >
+" URL: https://spacevim.org
+" License: GPLv3
+"=============================================================================
+
+function! SpaceVim#layers#lang#graphql#plugins() abort
+  let plugins = []
+  call add(plugins, ['jparise/vim-graphql', {'merged' : 0}])
+  return plugins
+endfunction

docs/layers/lang/graphql.md

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+---
+title: "SpaceVim lang#graphql layer"
+description: "This layer adds graphql file support to SpaceVim"
+---
+
+# [Available Layers](../../) >> lang#graphql
+
+<!-- vim-markdown-toc GFM -->
+
+- [Description](#description)
+- [Features](#features)
+- [Install](#install)
+
+<!-- vim-markdown-toc -->
+
+## Description
+
+This layer adds graphql file support to SpaceVim.
+
+## Features
+
+- syntax highlighting
+
+## Install
+
+To use this configuration layer, update custom configuration file with:
+
+```toml
+[[layers]]
+  name = "lang#graphql"
+```