You can now use our developer-documentation style guide for open source documentation projects.
The style guide is a reference document, so instead of reading through it in linear order, you can use it to look things up as needed. For matters of punctuation, grammar, and formatting, you can do a search-in-page to find items like “Commas,” “Lists,” and “Link text” in the left nav. For specific terms and phrases, you can look at the word list.
If you’re a developer this is a great place to start.
You can access Google style guide here.