Give contributors some maybe-adequate information [#12]

I honestly have no idea how useful this document is, but it's probably
better than nothing?

.drone.yml

@@ -30,3 +30,12 @@ steps:
       dockerfile: Dockerfile
     when:
       event: [ tag, push ]
+
+  # Example configuration for publishing to a local registry for testing.
+  # - name: publish_locally
+  #   image: plugins/docker
+  #   settings:
+  #     dockerfile: Dockerfile
+  #     insecure: true
+  #     registry: 0.0.0.0:5000
+  #     repo: 0.0.0.0:5000/drone-helm3

docs/contributing.md

@@ -0,0 +1,48 @@
+# Contributing to drone-helm3
+
+We're glad you're interested in contributing! Here are some guidelines that will help make sure everyone has a good experience:
+
+## Submitting a patch
+
+Before you start working on a change, please make sure there's an associated issue. It doesn't need to be thoroughly scrutinized and dissected, but it needs to exist.
+
+Please put the relevant issue number in the first line of your commit messages, e.g. `vorpalize the frabjulator [#42]`. Branch names do not need issue numbers, but feel free to include them if you like.
+
+We encourage you to follow [the guidelines in Pro Git](https://git-scm.com/book/en/v2/Distributed-Git-Contributing-to-a-Project#_commit_guidelines) when making commits. In short:
+
+* Commit early and commit often.
+* Make the first line of the commit message concise--no more than 50 characters or so.
+* Make the rest of the commit message verbose--information about _why_ you did what you did is particularly helpful.
+
+Once you're satisfied with your work, send us a pull request. If you'd like, you can send the pull request _before_ you're satisfied with your work; just be sure to mark the PR a draft or put `[WIP]` in the title.
+
+## How to run the tests
+
+We use `go test`, `go vet`, and `golint`:
+
+```
+go test ./cmd/... ./internal/...
+go vet ./cmd/... ./internal/...
+golint -set_exit_status ./cmd/... ./internal/...
+```
+
+If you have [the Drone cli tool](https://docs.drone.io/cli/install/) installed, you can also use `drone exec --include test --include lint`.
+
+## Using the plugin locally
+
+The internal tests can't test drone-helm3's integration with drone and helm themselves. However, you can build and run a local image to test a change end-to-end.
+
+You will need:
+
+* A Docker image registry. See [docs.docker.com/registry/](https://docs.docker.com/registry/) for information on standing up a local registry.
+* You will also need [the Drone cli tool](https://docs.drone.io/cli/install/).
+* A fixture repo--a repo with a `.drone.yml` and a helm chart.
+* Access to a kubernetes cluster.
+
+Once you have a local registry, uncomment the `publish_locally` step in `.drone.yml` and replace the `0.0.0.0`s with your computer's local IP address.
+
+Now you can run `drone exec --include test --include lint --include publish_locally` to build an image and publish it to your local registry.
+
+Finally, configure your fixture repo to use the locally-published image, e.g. `image: 192.168.0.1:5000/drone-helm3`.
+
+Now you can use `drone exec` in the fixture repo to verify your changes.