diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 0000000..5cd3e51
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,76 @@
+# Contributing to Starlette
+
+The Starlette team happily welcomes contributions. This document will help you get ready to contribute to Starlette!
+
+> **NOTE**: before writing any code, please first [open an issue] and discuss your ideas with the maintainers.
+
+## Setting up the repository
+
+1. Fork the repo.
+2. Clone your fork on your local computer: `git clone https://github.com/<username>/starlette.git`.
+3. Install Starlette locally and run the tests (see next sections).
+4. Create a branch for your work, e.g. `git checkout -b fix-some-bug`.
+5. Remember to include tests and documentation updates if applicable.
+6. Once ready, push to your remote: `git push origin fix-some-bug`.
+ 7. [Open a PR] on the main repo.
+
+## Install
+
+- Make sure you are using a compatible Python version, as specified in our README.
+- Create a virtual environment and install project dependencies:
+
+```bash
+python -m venv venv
+source venv/bin/activate
+pip install -r requirements.txt
+```
+
+- Install an editable version of the project: `pip install -e .` (see [Editable Installs] in the `pip` docs).
+
+## Running the tests
+
+The tests are written using [pytest] and located in the `tests/` directory.
+
+**Note**: tests should be run before making any changes to the code in order to make sure that everything is running as expected.
+
+We provide a stand-alone **test script** to run tests in a reliable manner. Run it with:
+
+```bash
+./scripts/test
+```
+
+By default, tests involving a database are excluded. To include them, set the `STARLETTE_TEST_DATABASE` environment variable to the URL of a PostgreSQL database, e.g.:
+
+```bash
+export STARLETTE_TEST_DATABASE="postgresql://localhost/starlette"
+```
+
+## Linting
+
+We use [Black] as a code formatter. To run it along with a few other linting tools, we provide a stand-alone linting script:
+
+```bash
+./scripts/lint
+```
+
+If linting has anything to say about the code, it will format it in-place.
+
+To keep the code style consistent, you should apply linting before committing.
+
+## Documentation
+
+The documentation is built with [MkDocs], a Markdown-based documentation site generator.
+
+To run the docs site in hot-reload mode (useful when editing the docs), run `$ mkdocs serve` in the project root directory.
+
+For your information, the docs site configuration is located in the `mkdocs.yml` file.
+
+Please refer to the [MkDocs docs][MkDocs] for more usage information, including how to add new pages.
+
+[open an issue]: https://github.com/encode/starlette/issues/new
+[Open a PR]: https://github.com/encode/starlette/compare
+[pytest]: https://docs.pytest.org
+[pytest-cov]: https://github.com/pytest-dev/pytest-cov
+[Black]: https://www.google.com/search?client=safari&rls=en&q=github+black&ie=UTF-8&oe=UTF-8
+[MkDocs]: https://www.mkdocs.org
+[Editable Installs]: https://pip.pypa.io/en/stable/reference/pip_install/#editable-installs
diff --git a/starlette/responses.py b/starlette/responses.py
index 542e800..3b8d275 100644
--- a/starlette/responses.py
+++ b/starlette/responses.py
@@ -63,8 +63,8 @@ class Response:
                 for k, v in headers.items()
             ]
             keys = [h[0] for h in raw_headers]
-            populate_content_length = b"content-length" in keys
-            populate_content_type = b"content-type" in keys
+            populate_content_length = b"content-length" not in keys
+            populate_content_type = b"content-type" not in keys
 
         body = getattr(self, "body", b"")
         if body and populate_content_length:

diff --git a/tests/test_responses.py b/tests/test_responses.py
index 0b79051..d4bafaf 100644
--- a/tests/test_responses.py
+++ b/tests/test_responses.py
@@ -275,3 +275,15 @@ def test_template_response():
 def test_template_response_requires_request():
     with pytest.raises(ValueError):
         TemplateResponse(None, {})
+
+
+def test_populate_headers():
+    def app(scope):
+        headers = {}
+        return Response(content="hi", headers=headers, media_type="text/html")
+
+    client = TestClient(app)
+    response = client.get("/")
+    assert response.text == "hi"
+    assert response.headers["content-length"] == "2"
+    assert response.headers["content-type"] == "text/html; charset=utf-8"

Contributing guidelines
Hi!

While figuring out a potential solution for #289, I had to setup the repo and install Starlette locally.

Although the process is rather straight-forward, I think it would be good practice to have contribution guidelines to make things clear and facilitate contributor onboarding.

I already went ahead and committed a [draft](https://github.com/encode/starlette/commit/df8805aa1ba26e46d90673a53e7029bc113380b7) to my fork based on the guidelines I wrote for Bocadillo. If it looks fine, I'd be happy to open a PR. :+1:
tests/test_responses.py::test_populate_headers
tests/test_responses.py::test_text_response tests/test_responses.py::test_bytes_response tests/test_responses.py::test_ujson_response tests/test_responses.py::test_redirect_response tests/test_responses.py::test_streaming_response tests/test_responses.py::test_response_headers tests/test_responses.py::test_response_phrase tests/test_responses.py::test_file_response tests/test_responses.py::test_file_response_with_directory_raises_error tests/test_responses.py::test_file_response_with_missing_file_raises_error tests/test_responses.py::test_set_cookie tests/test_responses.py::test_delete_cookie tests/test_responses.py::test_template_response tests/test_responses.py::test_template_response_requires_request