Add Danger bot development documentation

This is overdue, and some of the suggestions are embarrassing, but
this is how we actually develop Danger today.

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+# Danger bot
+
+The GitLab CI pipeline includes a `danger-review` job that uses [Danger](https://github.com/danger/danger)
+to perform a variety of automated checks on the code under test.
+
+Danger is a gem that runs in the CI environment, like any other analysis tool.
+What sets it apart from, e.g., Rubocop, is that it's designed to allow you to
+easily write arbitrary code to test properties of your code or changes. To this
+end, it provides a set of common helpers and access to information about what
+has actually changed in your environment, then simply runs your code!
+
+If Danger is asking you to change something about your merge request, it's best
+just to make the change. If you want to learn how Danger works, or make changes
+to the existing rules, then this is the document for you.
+
+## Operation
+
+On startup, Danger reads a [`Dangerfile`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/Dangerfile)
+from the project root. GitLab's Danger code is decomposed into a set of helpers
+and plugins, all within the [`danger/`](https://gitlab.com/gitlab-org/gitlab-ce/tree/master/danger/)
+subdirectory, so ours just tells Danger to load it all. Danger will then run
+each plugin against the merge request, collecting the output from each. A plugin
+may output notifications, warnings, or errors, all of which are copied to the
+CI job's log. If an error happens, the CI job (and so the entire pipeline) will
+be failed.
+
+On merge requests, Danger will also copy the output to a comment on the MR
+itself, increasing visibility.
+
+## Development guidelines
+
+Danger code is Ruby code, so all our [usual backend guidelines](README.md#backend-guides)
+continue to apply. However, there are a few things that deserve special emphasis.
+
+### When to use Danger
+
+Danger is a powerful tool and flexible tool, but not always the most appropriate
+way to solve a given problem or workflow.
+
+First, be aware of GitLab's [commitment to dogfooding](https://about.gitlab.com/handbook/engineering/#dogfooding).
+The code we write for Danger is GitLab-specific, and it **may not** be most
+appropriate place to implement functionality that addresses a need we encounter.
+Our users, customers, and even our own satellite projects, such as [Gitaly](https://gitlab.com/gitlab-org/gitaly),
+often face similar challenges, after all. Think about how you could fulfil the
+same need while ensuring everyone can benefit from the work, and do that instead
+if you can.
+
+If a standard tool (e.g. `rubocop`) exists for a task, it is better to use it
+directly, rather than calling it via Danger. Running and debugging the results
+of those tools locally is easier if Danger isn't involved, and unless you're
+using some Danger-specific functionality, there's no benefit to including it in
+the Danger run.
+
+Danger is well-suited to prototyping and rapidly iterating on solutions, so if
+what we want to build is unclear, a solution in Danger can be thought of as a
+trial run to gather information about a product area. If you're doing this, make
+sure the problem you're trying to solve, and the outcomes of that prototyping,
+are captured in an issue or epic as you go along. This will help us to address
+the need as part of the product in a future version of GitLab!
+
+### Implementation details
+
+Implement each task as an isolated piece of functionality and place it in its
+own directory under `danger` as `danger/<task-name>/Dangerfile`.
+
+Add a line to the top-level `Dangerfile` to ensure it is loaded like:
+
+```ruby
+danger.import_dangerfile('danger/<task-name>')
+```
+
+Each task should be isolated from the others, and able to function in isolation.
+If there is code that should be shared between multiple tasks, add a plugin to
+`danger/plugins/...` and require it in each task that needs it. You can also
+create plugins that are specific to a single task, which is a natural place for
+complex logic related to that task.
+
+Danger code is just Ruby code. It should adhere to our coding standards, and
+needs tests, like any other piece of Ruby in our codebase. However, we aren't
+able to test a `Dangerfile` directly! So, to maximise test coverage, try to
+minimize the number of lines of code in `danger/`. A non-trivial `Dangerfile`
+should mostly call plugin code with arguments derived from the methods provided
+by Danger. The plugin code itself should have unit tests.
+
+At present, we do this by putting the code in a module in `lib/gitlab/danger/...`,
+and including it in the matching `danger/plugins/...` file. Specs can then be
+added in `spec/lib/gitlab/danger/...`.
+
+You'll only know if your `Dangerfile` works by pushing the branch that contains
+it to GitLab. This can be quite frustrating, as it significantly increases the
+cycle time when developing a new task, or trying to debug something in an
+existing one. If you've followed the guidelines above, most of your code can
+be exercised locally in RSpec, minimizing the number of cycles you need to go
+through in CI. However, you can speed these cycles up somewhat by emptying the
+`.gitlab/ci/rails.gitlab-ci.yml` file in your merge request. Just don't forget
+to revert the change before merging!
+
+You should add the `~Danger bot` label to the merge request before sending it
+for review.
+
+## Current uses
+
+Here is a (non-exhaustive) list of the kinds of things Danger has been used for
+at GitLab so far:
+
+- Coding style
+- Database review workflow
+- Documentation review workflow
+- Merge request metrics
+- Reviewer roulette workflow
+- Single codebase effort
+
+## Limitations
+
+- [`danger local` does not work on GitLab](https://github.com/danger/danger/issues/458)
+- Danger output is not added to a merge request comment if working on a fork.