Add note on translation to readme file

Author: marijnh

README.md

@@ -7,10 +7,47 @@ Feedback welcome, in the form of issues and pull requests.
 
 ## Building
 
+This builds the HTML output in `html/`, where `make` is GNU make:
+
     npm install
     make html
 
-To build the PDF file:
+To build the PDF file (don't bother trying this unless you really need
+it, since this list has probably bitrotted again and getting all this
+set up is a pain):
 
     apt-get install texlive texlive-xetex fonts-inconsolata fonts-symbola texlive-lang-chinese inkscape
     make book.pdf
+
+## Translating
+
+Translations are very much welcome. The license this book is published
+under allows non-commercial derivations, which includes open
+translations. If you do one, let me know, and I'll add a link to the
+website.
+
+A note of caution though: This text consists of about 130 000 words,
+the paper book is 400 pages. That's a lot of text, which will take a
+lot of time to translate.
+
+If that doesn't scare you off, the recommended way to go about a
+translation is:
+
+ - Fork this repository on GitHub.
+
+ - Create an issue on the repository describing your plan to translate.
+
+ - Translate the `.md` files in your fork. These are
+   [CommonMark](https://commonmark.org/) formatted, with a few
+   extensions. You may consider omitting the index terms (indicated
+   with double parentheses and `{{index ...}}` syntax) from your
+   translation, since that's mostly relevant for print output.
+
+ - Publish somewhere online or ask me to host the result.
+
+Doing this in public, and creating an issue that links to your work,
+helps avoid wasted effort, where multiple people start a translation
+to the same language (and possibly never finish it). (Since
+translations have to retain the license, it is okay to pick up someone
+else's translation and continue it, even when they have vanished from
+the internet.)