Add a style guide (#591)

This is an attempt at documenting some things to do and don’t do when
updating the course text or a translation.

Fixes #560.

STYLE.md

@@ -0,0 +1,89 @@
+# Comprehensive Rust 🦀 Style Guide
+
+The course has been expanded and improved by tons of volunteers like you! Thank
+you for that! To help ensure a consistent style throughout the course, we have
+written down some guidelines for you to follow.
+
+## Course Slides
+
+Please take the following into account when updating the course material.
+
+### Vertical Space
+
+What looks like pages in a browser, are actually slides in a presentation. It is
+important to keep this in mind when adding content: we only have limited
+vertical space. Scrolling up and down should be avoided since it is very jarring
+for people who attend the class.
+
+You can test the amount of space available using a simple tool. Uncomment these
+lines in the `book.toml` file to have a red rectangle rendered on top of all
+pages:
+
+```toml
+[preprocessor.aspect-ratio-helper]
+command = "./aspect-ratio-helper.py"
+```
+
+The rectangle has an aspect ratio similar to what you can see when you share
+your screen on a 16:9 display or projector.
+
+Use the rectangle as a rough guide for how much you can fit on a single slide.
+If you find yourself adding too much detail, move the details to the speaker
+notes (see below).
+
+### Rust Code
+
+When showing Rust code, please use the same spacing as `rustfmt`: `3 * x`
+instead of `3*x`. However, feel free to remove newlines when it can make the
+code more compact and easier to understand, e.g., you can use
+
+```rust
+struct Person { name: String }
+```
+
+if you like.
+
+## Speaker Notes
+
+We have extended `mdbook` with support for speaker notes: content added between
+`<details> ... </details>` tags is rendered in a special box that can be
+collapsed or removed entirely from the slide.
+
+- The speaker notes should expand on the topic of the slide. Use them to provide
+  interesting background information for both the instructor and for students
+  who look at the material outside of a class. Remember that many more people
+  will read the course by themselves, so make the notes complete and useful even
+  when there is no Rust expert around.
+
+- Avoid using speaker notes as a script for the instructor. When teaching the
+  course, instructors will only have time to glance at the notes so it is not
+  useful to include full paragraphs which the instructor should read out loud.
+
+## Translations
+
+This section is about what you write in the translation. We describe [how to
+create or update translations elsewhere](TRANSLATIONS.md).
+
+When translating the course, please take the following into account:
+
+- Do not translate the course name ("Comprehensive Rust 🦀"). If the name is not
+  easily understood in your language, please add the translated version after
+  the original name.
+
+- If the Rust Book has been [translated into your
+  language](https://doc.rust-lang.org/book/appendix-06-translation.html), please
+  use the same vocabulary.
+
+- Be careful to preserve the Markdown syntax of the original text. Pay special
+  attention to reference links in all their variations: `[foo][bar]`, `[foo][]`
+  (which means `[foo][foo]`), and `[foo]` (which also means `[foo][foo]`).
+
+  As an example, if you translate `[foo]`, to `[FOO]`, you must also update the
+  corresponding link definition from `[foo]: https://example.net` to `[FOO]:
+  https://example.net`. If you forget to do this, you end up with a broken link
+  in the translation.
+
+- If you find mistakes or things that sound awkward in the original English
+  text, please submit PRs to fix them! Fixing typos in the translation is great,
+  but we want everybody to benefit from the fixes and that is why we need the
+  fix to be made in the English text too.

TRANSLATIONS.md

@@ -46,6 +46,9 @@ find mistakes, you need to update the original English text instead. The fixes
 to the English text will flow into the `.po` files the next time the translators
 update them.
 
+> **Tip:** See our [style guide](STYLE.md) for some things to keep in mind when
+> writing the translation.
+
 ### Generating the PO Template
 
 To extract the original English text and generate a `messages.pot` file, you run