docs: Add contribution guidelines document

This document is intended to provide some information about how to
contribute to the project, from basic sanity checks and code quality
to information about how to structure git commits.

These are the most common things that I encounter in pull requests
sent to my projects.

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+Contribution Guidelines
+=======================
+
+<!-- markdown-toc start - Don't edit this section. Run M-x markdown-toc-refresh-toc -->
+**Table of Contents**
+
+- [Contribution Guidelines](#contribution-guidelines)
+    - [Before making a change](#before-making-a-change)
+    - [Commit messages](#commit-messages)
+    - [Commit content](#commit-content)
+    - [Code quality](#code-quality)
+    - [Builds & tests](#builds--tests)
+
+<!-- markdown-toc end -->
+
+This is a loose set of "guidelines" for contributing to my projects.
+Please note that I will not accept any pull requests that don't follow
+these guidelines.
+
+Also consider the [code of conduct](CODE_OF_CONDUCT.md). No really,
+you should.
+
+## Before making a change
+
+Before making a change, consider your motivation for making the
+change. Documentation updates, bug fixes and the like are *always*
+welcome.
+
+When adding a feature you should consider whether it is only useful
+for your particular use-case or whether it is generally applicable for
+other users of the project.
+
+When in doubt - just ask me!
+
+## Commit messages
+
+All commit messages should follow the style-guide used by the [Angular
+project][]. This means for the most part that your commit message
+should be structured like this:
+
+```
+type(scope): Subject line with at most 68 a character length
+
+Body of the commit message with an empty line between subject and
+body. This text should explain what the change does and why it has
+been made, *especially* if it introduces a new feature.
+
+Relevant issues should be mentioned if they exist.
+```
+
+Where `type` can be one of:
+
+* `feat`: A new feature has been introduced
+* `fix`: An issue of some kind has been fixed
+* `docs`: Documentation or comments have been updated
+* `style`: Formatting changes only
+* `refactor`: Hopefully self-explanatory!
+* `test`: Added missing tests / fixed tests
+* `chore`: Maintenance work
+
+And `scope` should refer to some kind of logical grouping inside of
+the project.
+
+Please take a look at the existing commit log for examples.
+
+## Commit content
+
+Multiple changes should be divided into multiple git commits whenever
+possible. Common sense applies.
+
+The fix for a single-line whitespace issue is fine to include in a
+different commit. Introducing a new feature and refactoring
+(unrelated) code in the same commit is not fine.
+
+`git commit -a` is generally **taboo**.
+
+In my experience making "sane" commits becomes *significantly* easier
+as developer tooling is improved. The interface to `git` that I
+recommend is [magit][]. Even if you are not yet an Emacs user, it
+makes sense to install Emacs just to be able to use magit - it is
+really that good.
+
+For staging sane chunks on the command line with only git, consider
+`git add -p`.
+
+## Code quality
+
+This one should go without saying - but please ensure that your code
+quality does not fall below the rest of the project. This is of course
+very subjective, but as an example if you place code that throws away
+errors into a block in which errors are handled properly your change
+will be rejected.
+
+In my experience there is a strong correlation between the visual
+appearance of a code block and its quality. This is a simple way to
+sanity-check your work while squinting and keeping some distance from
+your screen ;-)
+
+## Builds & tests
+
+Most of my projects are built using [Nix][] to avoid "build pollution"
+via the user's environment. If you have Nix installed and are
+contributing to a project that has a `default.nix`, consider using
+`nix-build` to verify that builds work correctly.
+
+If the project has tests, check that they still work before submitting
+your change.
+
+Both of these will usually be covered by Travis CI.
+
+
+[Angular project]: https://gist.github.com/stephenparish/9941e89d80e2bc58a153#format-of-the-commit-message
+[magit]: https://magit.vc/
+[Nix]: https://nixos.org/nix/