message: Add test docs
diff: @@ -10,7 +10,7 @@ With GraphQL Modules and dependency injection it's much easier to test your modu
  So let's start with a basic module definition:
  -`modules/user/index.ts`
  +`modules/user/user.module.ts`
  ```typescript
  import { GraphQLModule } from '@graphql-modules/core';
  import gql from 'graphql-tag';
  @@ -47,3 +47,67 @@ export const UserModule = new GraphQLModule({
  },
  });
  ```
  +
  +You can mock context and providers by importing the existing module together with resolvers into a new testing module like below;
  +
  +tests/user.module.spec.ts
  +```typescript
  + import { UserModule } from '../modules/user/user.module';
  + import { execute } from 'graphql';
  + describe('UserModule', async () => {
  + it('FieldResolver of Query: me', async () => {
  + const { schema, context } = new GraphQLModule({
  + imports: [UserModule],
  + resolvers: UserModule.resolvers,
  + context: { currentUser: { id: 'ID', username: 'USERNAME' } }
  + });
  + const result = await execute({
  + schema,
  + document: gql`
  + query {
  + me {
  + id
  + username
  + }
  + }
  + `,
  + contextValue: await context({}),
  + });
  + expect(result.errors).toBeFalsy();
  + expect(result.data['me']['id']).toBe('ID');
  + expect(result.data['me']['username']).toBe('USERNAME');
  + });
  + it('FieldResolver of Query: userById', async () => {
  + const { schema, context } = new GraphQLModule({
  + imports: [UserModule],
  + resolvers: UserModule.resolvers,
  + providers: [
  + {
  + provide: UserProvider,
  + overwrite: true,
  + useValue: {
  + userById: (id: string) => { id, username: 'NAME' }
  + }
  + }
  + ]
  + });
  + const result = await execute({
  + schema,
  + document: gql`
  + query {
  + userById(id: "ANOTHERID") {
  + id
  + username
  + }
  + }
  + `,
  + contextValue: await context({}),
  + });
  + expect(result.errors).toBeFalsy();
  + expect(result.data['userById']['id']).toBe('ANOTHERID');
  + expect(result.data['userById']['username']).toBe('USERNAME');
  + });
  + });
  +```
  +
  +Then, run `jest` to get your test results.
  