Add README files to samples

Author: rstoyanchev

samples/README.md

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+This directory contains samples that we run to test different scenarios and features. 
+
+All samples use the Boot starter and expose a GraphiQL page where you can run queries. Simply start the `main` application class and go to http://localhost:8080/graphiql. 
+
+To run from the command line, use:
+```shell script
+$ ./gradlew :samples:{sample-directory-name}:bootRun
+```
+
+Samples have integration tests you can run with a debugger.
+

samples/webflux-security/README.md

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+Sample with Spring Security in a WebFlux application.
+
+Main features:
+
+ - Spring Security [config](src/main/java/io/spring/sample/graphql/SecurityConfig.java) secures GraphQL HTTP endpoint.
+ - Fine-grained, method-level security on [SalaryService](src/main/java/io/spring/sample/graphql/SalaryService.java).
+ - `AuthenticationException` and `AccessDeniedException` [resolved](src/main/java/io/spring/sample/graphql/SecurityDataFetcherExceptionResolver.java) to GraphQL errors.
+ - [Tests](src/test/java/io/spring/sample/graphql/SampleApplicationTests.java) with `WebGraphQlTester` and WebFlux without a server.

samples/webflux-websocket/README.md

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+Sample with GraphQL over WebSocket in a WebFlux application.
+
+Main features:
+
+ - Reactive [DataFetcher's](src/main/java/io/spring/sample/graphql/SampleWiring.java), including subscription stream.
+ - [WebFilter](src/main/java/io/spring/sample/graphql/ContextWebFilter.java) that inserts Reactor `Context` that is then accessed in the [DataRepository](src/main/java/io/spring/sample/graphql/DataRepository.java).
+ - [Tests](src/test/java/io/spring/sample/graphql/SubscriptionTests.java) for subscription streams. 

samples/webmvc-http-security/README.md

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+Sample with Spring Security in a Spring MVC application.
+
+Main features:
+
+ - Spring Security [config](src/main/java/io/spring/sample/graphql/SecurityConfig.java) secures GraphQL HTTP endpoint.
+ - Fine-grained, method-level security on [SalaryService](src/main/java/io/spring/sample/graphql/SalaryService.java).
+ - `AuthenticationException` and `AccessDeniedException` [resolved](src/test/java/io/spring/sample/graphql/SampleApplicationTests.java) to GraphQL errors.
+ - [Tests](src/test/java/io/spring/sample/graphql/SampleApplicationTests.java) with `WebGraphQlTester` and WebFlux without a server.

samples/webmvc-http/README.md

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+Sample with a GraphQL HTTP endpoint in Spring MVC.
+
+Main features:
+
+ - Spring HATEOAS [DataFetcher's](src/main/java/io/spring/sample/graphql/project/ProjectDataWiring.java) that call spring.io REST API.
+ - Querydsl [DataFetcher's](src/main/java/io/spring/sample/graphql/repository/ArtifactRepositoryDataWiring.java) making JPA queries.
+ - Use of [ThreadLocalAccessor](src/main/java/io/spring/sample/graphql/greeting/RequestAttributesAccessor.java) to propagate context to data fetchers.
+ - Schema printing enabled at "/graphql/schema".
+ - [Tests](src/test/java/io/spring/sample/graphql/project/MockMvcGraphQlTests.java) with `GraphQlTester` and MockMvc.