feat(docs): Include guide on CircleCI

Author: kylef

docs/index.rst

@@ -32,5 +32,6 @@ Integrations
    integrations/heroku
    integrations/travis-ci
    integrations/gitlab-ci
+   integrations/circle-ci
    integrations/docker
    integrations/cocoapods

docs/integrations/circle-ci.md

@@ -0,0 +1,44 @@
+# CircleCI
+
+You can use swiftenv in conjunction with Docker on
+[CircleCI](https://circleci.com) to easily test your Swift project on CircleCI.
+
+**NOTE:** *These instructions only cover using CircleCI on Linux and do not
+apply to CircleCI macOS containers.*
+
+### `Dockerfile`
+
+A `Dockerfile` may contain the instructions to build a docker container
+containing swiftenv, Swift and your source code.
+
+The following `Dockerfile` shows an example of setting up swiftenv and
+installing the version of Swift found in `.swift-version`.
+
+You may also base your image on top of `kylef/swiftenv:swift3` or
+`kylef/swiftenv:swift` to use pre-installed Swift versions.
+
+```
+FROM kylef/swiftenv
+
+RUN mkdir -p /code
+WORKDIR /code
+ADD . /code
+
+RUN swiftenv install
+```
+
+### `circle.yml`
+
+Using a `circle.yml` file we can instruct CircleCI to build and run `swift
+test` inside our docker container.
+
+```yaml
+machine:
+  services: docker
+
+dependencies:
+  override: docker build -t myapp .
+
+test:
+  override: docker run myapp swift test
+```