Add a PR template

.github/PULL_REQUEST_TEMPLATE.md

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+Describe your PR and link to any relevant issues. 
+
+I have:
+ - [ ] Added tests covering the bug / feature (see [testing](https://github.com/99designs/gqlgen/blob/master/TESTING.md))
+ - [ ] Updated any relevant documentation (see [docs](https://github.com/99designs/gqlgen/tree/master/docs/content))

TESTING.md

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+How to write tests for gqlgen
+===
+
+Testing generated code is a little tricky, heres how its currently set up.
+
+### Testing responses from a server
+
+There is a server in `codegen/testserver` that is generated as part 
+of `go generate ./...`, and tests written against it.
+
+There are also a bunch of tests in against the examples, feel free to take examples from there.
+
+
+### Testing the errors generated by the binary
+
+These tests are **really** slow, because they need to run the whole codegen step. Use them very sparingly. If you can, find a way to unit test it instead.
+
+Take a look at `codegen/input_test.go` for an example.
+
+### Testing introspection
+
+Introspection is tested by diffing the output of `graphql get-schema` against an expected output.
+
+Setting up the integration environment is a little tricky:
+```bash
+cd integration
+go generate ./...
+go run ./server/server.go
+```
+in another terminal
+```bash
+cd integration
+npm install
+SERVER_URL=http://localhost:8080/query ./node_modules/.bin/graphql get-schema
+```
+
+will write the schema to `integration/schema-fetched.graphql`, compare that with `schema-expected.graphql`
+
+CI will run this and fail the build if the two files dont match.
+